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.
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