KingDingaling,
You can request three Requested Days Off (RDO's) per month which once granted can not be changed as are any rostered days off once the roster is published. The roster is totally random comes out approx three weeks in advance and generally quite a lot of standby's, which of course they can either change to duty days before hand or obviously just call you out on.
This summer season haven't had too many changes or stand-by call outs but that is due to us being severely under-crewed this summer and being on max hours for the entire summer period.