Not sure if the OP is refering to a specific airline, but FWIW
I guess with most companies you need to take 14 days leave to get 14 days off,
Is certainly the way it works for full timers with us - we're on a form of "random" rosters (i.e. no fixed pattern).
if you book months in advance, surely you'll be using up holiday days when they would have given you the day off anyway?
In our case yup, correct.....you just have to suck it up.
How it works:
6-9 months ahead bid for leave...( 7 or 14 day block of guaranteed days off ).
Get leave bid result about a month later, only then can you book holidays and if you are sensible only book/pay for a holiday within those guaranteed days off.
Any other days around the leave block are an unknown until (relatively) the last minute - you might need to bid to work in them to create a valid roster, or the company might in some circumstances assign work into days around leave.