Transition is the seam between one month and the next.

The system wouldn't typically let you bid 2-3 weeks off straight in a given month, because you wouldn't be legal to cram a whole months worth of flying into a handful of days.

But if you load up the front of say May, and the back of the June you might well be able to bid a lot of days off at the end of May, say 7-10 and then do the same for the front of Jun.

You're still legal, although busy, in early May and late Jun but you got your required hours for both months and 2-3 weeks off too. No vacation involved.