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.
Somerset lost to Leicestershire in last year's final.