Today, I suggested includng the save the dates for our March 9 wedding in our Christmas cards, but my FI asked if that was tacky. I honestly have no idea. Thoughts?
"So what? So it's not going to be easy. It's gonna be hard.
Really hard. But I wanna do that because I want you
- all of you, you and me, every day. Forever."

Re: Rude to include Save the Dates in Christmas cards?
I also tend to think wedding information needs its own prominence-for example, let's say someone else in your family also has an occasion to celebrate, like a graduation. While it's more convenient to you, perhaps, to put everything in the same envelope, you run the risk that at the other end something gets jumbled/lost/misunderstood.
So don't use a Christmas card as your save-the-date. If you feel the need to send save-the-dates, do it separately from a Christmas mailing.