Actually, Set 17 will complete the Seven Set Block for this year. Only when Set 18 comes out do Sets 11, 12, and 13 rotate out. So we're all set until September.
So the way it actually works is that the minimum number of Sets in a Block is 5 and the maximum is 7. When the "Eighth" Set would be added, it kills the first 3 sets of that Block.
So..
Set 16 = Set 11-16
Set 17 = Set 11-17
ROTATION-
Set 18 = Set 14 - 18
Set 19 = Set 14 - 19
Set 20 = 14 - 20
ROTATION-
Set 21 = 17 - 21
Set 22 = 17 - 22
Set 23 = 17 - 23
ROTATION-
Set 24 = 20 - 24
etc