You can phone in to say you are off sick on Saturday as it's a rostered day off.
However, if you continue to be sick into your holiday legally you are allowed to claim any days back. Simply put you can't be on sick and annual leave at the same time. However, those days will count in the absence procedure. If you do feel better whilst on holiday make sure you contact them so they can change you from sick to annual leave.
Word of warning most RM managers won't be too clear on this process, or at least they won't advertise it, but it is your legal right.