A joke I like goes like this:
Q: What’s Santa Claus’s wife’s name?
A: Mary Christmas!
Recently, it struck me that that joke might work even better in those parts of the US that pronounce Merry and Mary identically.
But on second thoughts, I wonder whether it might not be better if the speaker does not have the merger, because then the pun is a bit clearer: it doesn’t sound as if the answer is merely “Merry Christmas” but you explicitly point at the given name “Mary”.
I’m still not sure which kind of accent (merging or non-merging) would be more effective for this pun.