Getting engaged, married, pregnant, or divorced might seem to have little to do with the season. But it could be better to plan these changes during certain months of the year.

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Some of the most profound decisions that you make in your life centre around family – from getting married to becoming a parent to even the best time to divorce your partner.

The seasons play a surprisingly strong role in these decisions, affecting our health, psychology and future prospects. The holiday calendar, too, dictates when is and isn’t a good time to be making a big decision.

While no two couples are the same, on a population level there are yearly patterns that emerge, showing the peak times to make important relationship decisions.

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BBC Future takes you through the calendar of when is most common – and when is best – to make big changes in your relationship.

Ring in the year

In the US, nearly 40% of engagements happen in the two-and-a-half months between Thanksgiving and Valentine’s Day, according to a WeddingWire survey of 18,000 people. December, in particular, is the peak month for getting engaged – with Christmas Eve a particularly popular date. A third of men in a Chillisauce survey of 10,000 people thought it was the best time to pop the question.

But men, take note: women tend not to agree. The same survey found the date that women thought was best to get engaged was Valentine’s Day, with 23% of votes. Unfortunately for them, the survey found that only 12% of proposals actually happened on Valentine’s Day.

In contrast Christmas Eve (the mens’ favourite) was the most common date, with 31% of proposals on 24 December – perhaps a reflection of which partner was the one to get down on one knee. New Year's Eve was the next most common choice, followed by New Year's Day and then an anniversary of when the couple first met.

While some surveys say the festive period is the most popular time, it’s less clear when is the best time – there’s no evidence that your engagement date has any bearing on the quality or length of your marriage. And, sadly, neither is there any data on the time of year that the apple of your eye is most likely to say yes.

Wedding bells

Did your other half say yes? Then it’s time to determine when to have the big day itself.

There are several factors to take into account. One is expense – the most popular time to get married in the US is autumn, while in the UK it is summer. In 2017 in the US, the most popular months for marriage were September (16%), June (15%) and October (14%), according to a survey of 13,000 brides and grooms by wedding-planning website The Knot.

Picking a date outside these times is likely to get you a better deal – which may be important for more reasons than your bank balance. A study of 3,000 US couples found that those who had a cheaper wedding ceremony were more likely to have longer marriages. After controlling for demographic and relationship characteristics, the researchers found that those in longer-lasting marriages also spent less on their engagement ring.

But the study was only observational, meaning that the authors could not draw any conclusions about cause and effect. In other words, it might not be the January wedding that helps couples last longer. Having a cheaper wedding may just be a sign of relationship health – perhaps the couple doesn’t feel the need to ‘prove’ their relationship to others with a big bash, or is focused on longer-term priorities that could signal deeper commitment.

The next factor to consider when picking a date is how old you and your partner will be. Tying the knot between the ages of 28 and 32 linked to the lowest levels of divorce five years later, according to a study by University of Utah sociology researcher Nick Wolfinger for the pro-marriage Institute for Family Studies.

Analysing data from more than 9,000 people, Wolfinger found that “prior to age 32 or so, each additional year of age at marriage reduces the odds of divorce by 11%. However, after that the odds of divorce increase by 5% per year”, he wrote in a blog post on his research.

The study suggests that there could be a ‘Goldilocks effect’ at work. Too young and we are still changing more year by year than later on in life, so marrying young may lead to later friction or growing apart. On the other hand, people who marry later in life may inherently be less suited to the institution, Wolfinger speculates. These people may somehow be more “congenitally cantankerous” and have trouble with their personal relationships, putting them at higher risk of divorce.

As there is no causal evidence to prove this, trying to time your wedding to fit into the sweet spot of 28-32 is probably not going to make you immune to divorce.

Quitting time

But what if it doesn’t work out — is there any ‘optimal’ month to call it quits?

In the US, one Washington-based study found that there were two peaks in divorce applications, in March and August. The reason behind this, the authors speculate, is that couples hold off over the holidays during winter and summer. These breaks are “symbolically charged moments” when filing for divorce would often be seen as inappropriate, the authors say. Or perhaps they’re simply impractical, as their lawyers may be out of the office too.

But once families and lawyers alike are back from their festivities, divorce applications surge. The reason the post-winter peak is delayed until March, the authors suggest, is because couples might take a while to get their resources together and hire a lawyer.

Beyond Washington state, US family lawyers anecdotally report an annual spike in couples seeking to end their marriages in January. Exactly why January is a popular moment isn’t clear, but it could be the combined effects of the end of the holiday period and the usual New Year urges for a fresh start.

How to hack your year

This story is part of a series we’re running on how to ‘hack’ your year. We’re looking at the best time of year to make a big decision, buy a house, sit an exam, go to hospital and more. Keep checking back through December here for more stories.

Or it could be a more prosaic reason: at least in the US, waiting until January has tax advantages. Divorces are often completed within a year, so filing for divorce in January can increase the odds of filing taxes as single person by the time the next tax year rolls around.

The jury is out, though, on whether filing at a certain time of year helps either party recover from the break-up faster.

Baby boom

What about starting a family? It may seem like birthdays are random – but in fact, there are more popular (and, in some ways, better) months of the year to give birth.

Several studies have shown that sperm are healthier in spring than at any other time of year

Take fertility. The quality and concentration of a man’s sperm is an important factor, and several studies have shown that sperm are healthier in spring than at any other time of year. One study, presented at the American Society for Reproductive Medicine conference in Denver in 2018, analysed sperm samples taken over a 17-year period from 29,000 men in the US. The men had the highest numbers of moving sperm during springtime – at 117 million per millilitre. The lowest levels were in the summer, at 112 million. And autumn was when these men had the greatest number of normally shaped sperm – another measure of sperm quality. Another study of 12,245 men in Switzerland found that sperm concentration was also highest in the spring, and lowest in the summer.

And samples collected in the morning had a higher sperm concentration and proportion of normally shaped sperm than those taken later in the day.

“The idea that spring is better for sperm is because it takes about three months to produce the sperm from start to finish,” says Allan Pacey, a professor of andrology at the University of Sheffield, who was not involved with either study. The sperm that are being ejaculated in spring were starting their life around Christmas, when it’s colder.”

Cool temperatures are good for sperm, which could suggest why sperm quality has a peak at this time. But other factors, such as the length of the day, also play a significant role in human fertility. Among other animals – such as sheep and cows – how many hours of light there are has a strong influence on when they are most fertile, to ensure their young are born with the best chances of surviving their first winter. It’s possible that daylight could also have a similar effect in humans, says Pacey, with long summer days linked to lower fertility.

Another reason that sperm quality could be low in the summer, Pacey says, is because we are having less sex. Several studies have found that while abstinence boosts the number of sperm cells, it also leaves sperm in significantly poorer shape, with poorer swimming ability and more DNA damage. Frequent sex, on the other hand, results in sperm that have a better shape, swim better, have less DNA damage and give rise to higher pregnancy rates.

But the changes noted in these studies, while significant, shouldn’t be relied upon to determine your calendar for trying to get pregnant, says Pacey. “A small change in sperm quality doesn’t necessarily translate to less chance of achieving pregnancy.”

Birthday boon

The best time of year for fertility is one thing, but what is the best time of year to be born?  

Conceiving in the spring, around March or April in the Northern Hemisphere at least, would lead to a full-term pregnancy delivering in December or January. But the best time to be born is actually a few months earlier – in September. Interestingly, September also is the most common month for birthdays in the US and in the UK.

Children born in August were twice as likely to be bullied in primary school

British children born in September tended to achieve better exam results than children born in August and had better cognitive skills, according to research from the Institute for Fiscal Studies, a UK think tank. Children born in August, on the other hand, were twice as likely to be bullied in primary school, 20% more likely to take vocational rather than academic qualifications at age 16-18, and 20% less likely to attend a top university. These effects were present regardless of a family’s socioeconomic status, suggesting the effect is pervasive and hard to shift.

The researchers later discovered that this effect wasn’t anything to do with the seasons – the same effect was seen across hemispheres, from Australia and Chile to Japan and the US. Instead, what all of these countries all have in common is the timing of the start of the academic year. The children’s cognitive skills were that much better if they were September-born because they were usually the oldest of their classmates. August-born children tended to be the youngest.

The ‘September effect’ persisted throughout their school career. At eight years old, September-born children had a higher sense of competence relative to their classmates, with the reverse true for August-born children.

Being born in autumn may also boost your longevity. A study of nearly 1,600 US centenarians found that people born between September and November were more likely to live to 100 than those born in March. This could be down to nutritional deficiencies during pregnancy or the seasonality of infections.

In temperate regions, babies born in the autumn avoid experiencing extreme temperatures right after birth, which can have surprisingly profound health consequences in later life. A UK study of more than 4,000 women found colder winter temperatures at birth was linked to a higher risk of coronary heart disease, insulin resistance and poor lung function in old age.

Women born in the coldest quarter of the year were 24% more likely to develop coronary heart disease

Women born in the coldest quarter of the year (based on actual temperatures in the year of their birth) were 24% more likely to develop coronary heart disease than women born in the other three-quarters of the year. The association was strongest for those whose fathers were unemployed or in manual jobs, suggesting that the effect may be down to these families struggling to heat their homes adequately.

We might like to think that every couple is unique. But the emerging trends in when people make their big decisions reveal the surprising extent to which external forces shape our relationships. Some of them seem incidental, like the holiday calendar influencing when people are most likely to turn their thoughts to romance and family. Others appear to be a product of our biology, like the fresh temperatures of spring lending a subtle boost to male fertility.

And as the September effect shows, the most robust trends happen when calendar events and the deeper biological effects of seasonal changes are in alignment.

Research by Miriam Quick.

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