Mum is annoyed because I've told her I'm not going to a party

Would you consider going alone? Your Mum may be hurt that everything is centred around your girlfriend when it is you she really wants to spend time with. Would she consider buying your ticket as a gift? Try and re-use a suit ,you already have.
She may seem annoyed, but really she is sad because she want to see you and celebrate Christmas with you.

I wouldn't go without my girlfriend, we live together so I am not leaving her home alone. The rest of my family will be going, my dad, both my sisters and their partners, I can understand she wants us there but unfortunately circumstances beyond our control are stopping us going. And I would have to buy or at least rent a black suit as the one I had previously no longer fits.

So some background info, my girlfriend worked at Wilko's however she has recently lost her job due to them going into administration. She has thankfully found another job but the pay isn't as good and the hours are awful but for the next few months we are just viewing the job as a means to an end to help us pay the bills. She has started her new job this week and has been told her shifts are 7-5, five days a week. She will have one day off during the week and one day over the weekend except for during the Christmas period when she will get two days off during the week and work all weekend. My job is 8-5 Monday to Friday so obviously in the build up to Christmas we wont be seeing much of each other.

We are going away on holiday for a week in a few weeks and thankfully her work have agreed to give her that and a weekend away we had already planned for late November. However after that it is unlikely she will be able to take any more time off this side of Christmas.

Last week my mum told the whole family she wanted us to go to a party she had seen advertised a few weeks before Christmas. It is a black tie event and is from 7 until the early hours. I have told my mum, me and my girlfriend are unlikely to go because 1. The tickets are expensive. 2. We will need to buy new outfits for the event when we are trying to save money as there will be less coming into the house. And 3. It starts at 6:30 on a day my girlfriend will have been working until 6 so we just will not have time to get ready and be there when it starts.

I have told my mum all that but she is still annoyed and trying to guilt trip us into going. I have told her that if we went we would arrive late and have to leave early as my girlfriend would be up before 6 the next day to go to work. My girlfriend have asked if there is anyway she could either have an early finish on the day of the party, or have the day after the party off but they have told her no. The only way she could have one day off would be an unpaid day off which is not something I would expect my girlfriend to give up.

My mum works for herself so can have any day off she fancies without having to run it by anyone but my girlfriend works in retail and my mum doesn't seem to understand that having a day off in retail is as easy as it sounds. My girlfriend has only been in this job for two weeks so she doesn't, understandably, want to get on the wrong side of her managers by asking for more time off.

Is there anything else I can do? I feel like I am caught in the middle here.