If We Don't Know the Birthday, How Can We Have a Party?

After we did our daily How many days to Christmas countdown, Middle Sister was informing me that she learned in school that "no one really knows what day Jesus was born. Maybe it wasn't the 25th. Maybe it was the 24th, or something."
(Or maybe not even in the winter, but I wasn't about to get into that with her at this point.)
She went on, "It would be really easy to find out, though. They should just check his tomb!"
Me: "He doesn't HAVE a tomb anymore, since he rose from the dead, and then ascended to Heaven."
Middle Sister: "OH YEAH!!!!!"