Some BG: DH and I had originally volunteered to host a family Christmas dinner on Dec. 15th, since we were going to be out of state on Christmas Day itself. Around Thanskgiving, our niece unexpectedly had to go to the hospital with a bone infection, and was projected to be there for six weeks, or until January 6th.
We rescheduled the family Christmas day to be January 13th, although one of my sisters in law kept saying we should reserve January 20th as well, "just in case". DH and I will still be hosting it, and I thought everything was set.
Found out this week that our niece only needed four weeks of treatment, and so was discharged last Friday. This is wonderful news! She gets to spend her first Christmas at home with her parents and big sister! Everybody was pretty excited about that, especially since we were told from the beginning it was always going to be six weeks.
Anyway, I got an email from my other sister in law yesterday, asking if we could move the family Christmas date again, to January 6th. So now we've agreed on the 13th, but one sister in law wants to "save" January 20th, and another wants to move everything to January 6th.

I know my social life is a bit dead, but I can't really reserve the entire month of January weekends! DH and I both have friends who would like to see us in January, and we've already told them any weekend except the 13-14th will work. In addition, DH might have to go in and do inventory at his work on the 6th. He's not totally sure though; it could be the weekend before instead. And, well, I'd like to have a bit more time to clean our house!
Flat out saying "I'm sorry, we can't switch the dates, we have other plans" will unleash a storm of guilt trippiness, from both sisters in law, plus my mother in law. Guilt trips are surprisingly effective on me, so I'd like to avoid that. Telling them that DH may have to work that day will be met with "oh, that's okay, we can come over later in the day for dinner then!". What are some nice, polite ways to say "No, we're having this on the 13th, like we already planned!".