What happens when Christmas day falls on a Sat. or Mon.?
Then we are still obligated to go both on Christmas Day and Sunday.
I presume you are asking about what happens to the feast day/celebration of the Mass.
The answer is “nothing”.
The Feast of the Nativity (a.k.a. Christmas) is never transferred or abrogated. Therefore, Dec 25 would be a holy day of obligation no matter what day it falls on.
The Sunday immediately before or after would also be a holy day of obligation in the case when Christmas is on a Saturday or Monday.
I thought so, but I couldn’t remember the last time Christmas fell on a Sat or Mon, what I did.