Probably being a bit dim here, but...
I work 18 hours a week, Mon, Tues and pm on Wednesdays. Over this Christmas week, staff could have gone into the office on Wed/Thurs/Fri if they wanted but it was closed as everyone took it as holiday.
My question is, since - apart from Wed pm - those aren't my normal work days, do they come out of my annual leave or not? i.e. because I didn't work this week will I have to give up 18 hours of holiday, or just the Wed pm, or anything at all???