There are several ways to try to get a particular time off.

Sequentially:

1) up to 2 months prior vacation swap
2) monthly bid with line/else start next, reserve/else start next
3) during the adjustment period IVD, APD, PD, swap with pot, pilot swap board
4) hardship need through chief pilot, usually not known in advance (bereavement etc.)
5) sick, usually not known in advance (people have been known to schedule surgery just prior to Thanksgiving and need until the new year to recover)










6) AWOL

None of these (save the last one) comes with any guarantee or percentage of success.