what does it say in your contract? are the days you currently do the ones that are written in your contract because if not then they can change your days to suit, alot of employers especially retailers such as tescos asda and morrisons trade over 7 days and need flexible staff, so as i the case of me when i worked at morrisons i had specific days on a contract but was expected to work other days as and when needed, check with union if unsure but check the wording on your contract too