Can I refuse to do a shift?

[postiewhite]

Im a PT reserve on 23.24hrs, our office has this over a 4 day working week although I usually do 5 and they nominate me one day a week to put down as a day out overtime. Anyway ive had one saturday off in 2015,this was brought up by the rep 7 weeks ago and I still havent had one. My rota next week has my day off as today bank holiday(which I know is common for reserves) but they have my other day which im allocated a duty is saturday which I find a cheek.
I havent agreed to work my other day off and they never ask me each week they just presume I will. If they did ask me I would have turned it down to have a rare saturday off!!
Can I do anything about this in the morning or just let them walk all over me? I will have done my contracted hrs by probably thursday/friday.

Completely failing anything as a plan B can I tell them I want today (bank hol monday) as an overtime day rather than take it in leiu as they already owe me 4 leiu days and they hard to take. I understand that was MY choice?

[Morning7]

Hey there, been a postie for 8 years and a ex deputy manager and current cwu rep in my office. If you don't want to work your day off then you don't need to, they can't assume you will do it (this is talking from experience), you should never feel as though you have to, you need a life outside of work aswell wether that be doing sumthin or nothin! As for the days in leui I'd be keeping a tab on them, they can be quite fly with days you are owed.

[postiewhite]

Thanks. I will. Owed 4 days as it stands!!! Ill be having a chat in the morning.

[Stella102]

go in and say sorry i can't work this saturday i've already made plans. It your day off - end of conversation you have given sufficient warning for them to sort it and the ball is in their court to deal with it - none of your concern once you've told them.