Me and my husband have decided to go away for Christmas how do we tell his mom who was expecting to come to ours for dinner?

Son needs to have this type of conversation with his mom:

Mom, this year, Carol and I have decided to change things up in regards to celebrating Christmas. Our plan is to go to Cancun on December 22nd and return around New Year's Day.

What we'd like to do is celebrate our Christmas holiday with you on Thursday, December 21st. We would host a dinner for you (and anyone else you want to invite) and exchange gifts then.

It would be natural for you to feel sad and disappointed that we have chosen to change things up a bit for the holidays, but we certainly want to celebrate that holiday with you - just not on the actual Christmas Day. Would you please let me know if that date will work for you?

Keep the tone matter-of-fact. Don't let mom guilt you into changing your plans. Offer up a date to celebrate this holiday with her. Good luck!

This actually happened to us several years ago.  Our son and daughter in law ask to talk. They told us they knew we expected the family at our house on Christmas. But, they wanted to take the two boys and go away together for Christmas. We both looked at each and almost at the same time said, you do what you need to do for your family.  We will miss you but we understand. They went and we had Christmas with them after the new year.