My fourpence worth on the stuff not specific to shorthaul.
Leave is allocated twice a year (two "seasons"). Keeping it simple you get a couple of weeks in the "summer", couple of weeks in the "winter" season. You also get a Duty Free Week in each season...Roughly speaking the lead time for leave is around 6 months (e.g. bid in the Spring for winter leave), lead time for a DFW 2-3 ish months minimum (e.g. we're bidding now for the winter DFWs). There's a points system to ensure that the senior pilots don't perpetually hoover up the attractive dates.
Ability to get certain days off in advance (i.e. 4-6 weeks)
Aside from compassionate reasons and possibly days off associated with a reserve block there's effectively Nil/zero/nada official mechanism in place to guarantee a specified single day off on a full time contract. Obviously (?) your first option is to frame a suitable monthly bid around the day in question, but even if you do manage to be successful at the end of the rostering process you might still up being Force Assigned (FA) extra work, possibly neatly clobbering that special day off that you thought you'd managed to keep clear. About the only hope than is that you can find someone to swap with.
If you really need a day off you need to embed it in a leave block or a DFW - and you can't do that with 4-6 weeks notice.