Scarlets will face champions Glasgow Warriors in the opening round of matches of the 2015-16 Pro12 season.
Wayne Pivac's side, who finished sixth in 2014-15, travel to Scotstoun on Saturday, 5 September.
Ospreys kick off the new Pro12 season away at Ulster on Friday, 4 September in Scrum V Live's first match of the season.
The regions will be missing many Wales players in Warren Gatland's 31-man Rugby World Cup squad.
The Rugby World Cup kicks off on 18 September, with Gatland's original 47-man squad due to be whittled down on 31 August.
The west Wales region potentially have another five players missing with the Wales squad, while Ospreys could have a dozen absent including half-backs Dan Biggar and Rhys Webb.
Cardiff Blues will definitely be without the services of Wales captain Sam Warburton plus up to eight others, while Newport Gwent Dragons could have four - assuming that British and Irish Lions number eight Taulupe Faletau - linked strongly with Bath - is still with the region.
Glasgow Warriors have 22 players with an expanded 46-man initial Scotland squad, with Edinburgh providing 12.
The four Irish provinces are likewise affected, as Ireland's 45-man expanded squad has 22 Munster players, 11 from Ulster, nine from Munster and two from Connacht.
The two Italian sides contribute 26 players to Jacques Brunel's provisional 40-man training squad, 12 from Treviso and 14 from Zebre.
Zebre fly-half Kelly Haimona was named in the squad, but has since withdrawn due to problems with the arm he broke in the final Six Nations game against Wales.
Like Wales, the other competing nations will trim their squads to 31 on 31 August.
The availability of players to their teams will also depend on World Cup progress beyond the group stages.
Wales' final Pool A game is against Uruguay on 10 October with the four quarter-finals played on 17 and 18 October and the semi-finals a week later. The final will be held at Twickenham on 31 October, the same weekend as round six of Pro12 matches.
In the Pro12, Steve Tandy's Ospreys side were the only Welsh region to reach last season's play-offs but lost to Munster in the semi-finals.
Munster, beaten by Glasgow in last season's play-off final, face Treviso at home in the opening game while Edinburgh host Leinster.
Blues, under new head coach Danny Wilson, will host last season's bottom-placed side Zebre on Sunday, 6 September in a match televised live by S4C.
Dragons will be away at Connacht on the opening weekend with Lyn Jones' side away to Scarlets in the first Welsh derby of the season on Friday, 30 October.
Meanwhile, Dragons will host Cardiff Blues on Saturday, 27 December a day after the Boxing Day clash between Scarlets and Ospreys, which will be shown on Scrum V Live.