I feel your pain mate. I started working in Supermarket i work in in September 2010, I just blagged that i could work any day in the interview, and then when i got it, they slapped me on a saturday 11-9.30, which as you may be aware, ruins your day.
2 years on, changed it to a 12-6, which again meant i missed the footie. Being the crafty [Poor language removed] that i am, i pulled a sickie last year so i could go to Stoke away of all places, and got a tap on my shoulder 5 minutes before the game... 'Ay, thought you were sick' my manager behind me, was officially the worst 90 minutes of my life, pulled into the office on my next shift, and had to fess, got a warning, to add to my other warning, including one for watching Everton-Spurs on SkyGo on my phone, instead of serving customers
But yeah, if you're telling yourself that you could probably do it, then have a hard think about it, any doubt then either say when you signed the contract you agreed to do them hours, and that the hours you've been given are inaccessible, if that doesn't work then look for a new job.
I finally got my weekends back, and it feels great. Got promoted too, must be all those excuses 'Got a field trip to Fulham on a Tuesday night, can't work sorry'