Could you imagine wishing someone a “Cheery Christmas”? 100 years ago the argument was that merriment was associated with bad behavior and drunkenness so we should say Cheery instead! That would really change a lot of our Christmas songs! We Wish You a Cheery Christmas just doesn’t sound right!
Either way…I hope you have a CHEERY CHRISTMAS or a MERRY CHRISTMAS if you are planning on getting into some bad behavior!