I have worked for the company for five years, taken on, on a part time basis – normal working days Wednesday, Thursday & Friday office hours. I do holiday cover for two colleagues if they have Mondays and Tuesdays off as extra to my normal working days . This is fine in general. However upon sorting out 2020 holidays I have stated that on certain Mondays and Tuesdays I cannot cover. My manager has said that it is not right that I can’t work those days. Can she enforce me to not make plans on my days off? In the contract it states that my holidays have to be agreed, but there is not anything that says I have to cover holidays – although I do do it.