Can my manager make me change my availability?
hi, i've tried to look for this answer on my own but it's been a little confusing.
I started working at my job a little over a year ago (part time but the hours i get are basically full time) and from the very start (i.e written on my application) i've had sundays as not available. When they asked me to change my hours to be earlier or later i've readily agreed but sundays has been my hard "no, sorry, i'm not available then."
Today my manager approached me while i was with a customer and asked why i wasn't available on sundays. I was a bit caught off because it was a customer but gave her a basic "the bus schedule is weird and that's the day i schedule stuff." I am not the only one to not work on sundays which apparently has left them without many options. She told me she needed me to change my availability in the app to be available sundays, i said "do i have to?" and she said something to the effect of "well no, i just need you to change it. For the needs of the buisness." I ended up having to go back to actually doing my job so we didn't really have a conclusion to the conversation and i overheard her having the same conversation with another coworker.
So can someone tell me in plain terms; do i have to do this? Is she allowed to demand this of me or can i say no and refuse?