Part time work

I work part time. Recently my workplace closed for a private function on a day that I usually work. I haven't been paid for that day, I thought being part time I would need to be?

UPDATE
I queried it and she told me that she advised everybody that the function was booked more than 6 months ago and she told everybody at that point that no Monday-Thursday hours will be paid that week if not worked. Unfortunately I was on maternity leave when she advised this.