Individuals leave can be booked as and when, but most RM units still take block leave periods that overlap so some ranks will be on camp - this is known as "rear party" xmas and easter are good examples of this summer leave tend to be a bit more spread out, but not always.
Units hold something called a long cast which is a 2-year training plan, and if you know your unit/company is due to be away in Oman or somewhere else you will not normally be allowed to place leave in over that period, the lads only really have oversight over the next few months maybe, so asking your troop Sgt or CSM is a good idea if planning leave a long way out.
Even if you do ask and plan well ahead, the powers that be can on occasion try to backtrack on the "promised" leave, so if that happens you have to fight your corner or a good NCO / SNCO may do it for you. Most places i have been to try to help if you have something booked such as a wedding or long time holiday booked , but to put it in a nutshell junior ranks should always check as their leave needs to be approved anyway , only SNCO and above approve requires no approval.
