My Family Postponed Christmas. What do I do?

With the big snow storm that just ripped through this part of the US my grandparents were himming and hawing about whether or not to postpone Christmas until the Sunday following (the one directly after Christmas). On Christmas eve in the afternoon it was decided that it would be postponed until Sunday. Christmas day came and went (it was 40 degrees up here and beautiful). Several people and I had discussed our disappointment with having Christmas moved up, my parents who had made food for the dinner and prepared it the day before, my sister and brother in law who had also made food for the dinner were all unhappy with the decision to postpone it. Everyone who was going to the dinner was bringing something to contribute and I had purchased over 50$ worth of food to bring (being unemployed that is A LOT of money). I had also found out, after finding out what day my family had moved X-mas to, that my fiancee had made plans for us to go to her mothers Christmas dinner on the same day. The original plan was to do X-mas with her mom on the 20th and her sister said no. We couldn't do it on X-mas day due to her sister being out of town, so I thought we had decided to do it on the 3rd of January, which was fine with me. But now as soon as I find out what day my family's X-mas was on, I find out that her mom had moved it ahead to the 27th, the same day. My dinner was supposed to be early (around 1:00 P.M.) and her mom's dinner is supposed to be around dinner time (5:00 or 6:00 P.M.) so I figured we could make it to both. I was wrong. I asked my fiancee if we got to my family's place around 12:00 noon and were there until around 2:30 or 3:00 P.M. and then went to her mom's place which I thought was perfectly reasonable. She told me that we were absolutely expected at her mother's at 12:00 noon and wouldn't be able to make it to my family's Christmas at all.

Now my question is what should I do? I have tried to be reasonable and accommodating to everyone else's schedules and have desperately tried to compromise with her mother, but have been told that there is an entire DAY of events planned for that day by her. I'm conflicted! I feel the inherent obligation to attend my family's X-mas dinner not only because I have been for over 25 years, or because I have over 50$ worth of food that is going to go to waste, but also because not only for me, but for the 50+ other people attending my family's X-mas, this is the one and only time of the year that all of my family gets together which is why I look forward to it so much.

Now, on the other hand, I can also understand skipping my family's dinner. My family did re-schedule at the last minute and we had promised to have christmas another time with her mom. It all stems from a promise we made. I thought it was on the 3rd of Jan, but apparently it's on the 27th. I have a lot of confusion and misplaced anger about all of this and have been taking it out on my fiancee. I'm frustrated with my family for re-scheduling X-mas, BUT I'm also mad that I committed to doing something with her mom without having a specific day set in stone, leaving it between her sister and her mom to conveniently change the day of the dinner to the same day as my family's dinner.

Part of me just wants to just go to my family's dinner out of spite because I have had issues with her mom telling us what to do or where to go on the holidays.

The other part says go to her mom's and forget about everyone at my family's dinner because of their lack of consideration to other people's schedules and for being indecisive and changing the plans at the last minute.....

HELP!!!