Last season's runners-up Somerset will host Essex in the first of this year's FL t20 quarter-finals on Tuesday, 24 July (16:15 BST).
Sussex, winners in 2009, will entertain Gloucestershire the same day in a day-night encounter at Hove (19:10 BST).
North Group winners Yorkshire play Worcestershire at 16:15 BST on 25 July.
Four-time quarter-finalists Nottinghamshire host 2010 champions Hampshire under the lights at Trent Bridge later that day (19:10 BST).
The four winners will qualify for finals day, which will this year be held at the Swalec Stadium, Cardiff, on Saturday, 25 August.