Why is it offensive to say Merry Christmas?
Because saying โmerryโ in England can mean that person is a drunk. Or wanton! Or they give in to indulgences. INDULGENCES!
Charles II (reigned 1660-85) was the โmerry monarchโ not because he was happy but because he was overly indulgent and exuberant in everything. Unlike the current Charles.

But this does explain why many English people say โHappy Christmasโ instead of โMerry Christmas.โ They donโt want to encourage you to drink. Especially not at their house. And especially not their mulled wine! Apparently genteel Brits in the 19th century switched to โHappy Christmasโ so they wouldnโt encourage all this low class behaviour. SO be genteel if you want to be genteel.
I, for one, am very merry at Christmas time in terms of indulgences--like if you offer me a Smartie cookie I will indulge in it. I will also indulge in short bread and gingerbread and nut squares and toffee popcorn and those crunchy cerials mixed with nuts and I also indulge in turkey and mashed potatoes and gravy and wellโฆ you get the picture.
So may you have a Merry Christmas, a Merry Holiday, a Merry Celebration of your own choosing, religious or otherwise, and a very Merry New Year. And a Happy New Year, too.
And may you read many merry books. I mean ones that make you feel merry, not ones that make you feel like drinking.
Thanks for reading!
P.S. I know, I know, Iโm supposed to promote my novels. My apologies for being not being commercial in this most commercial of seasons. I do encourage you to go out and gift as many books as possible. It will make other people merry.
P.P.S. This whole โmerryโ thing makes me wonder why Santa has such red, red cheeksโฆ
To be fair, "merry" is also the name of one of J.R.R. Tolkien's hobbits, as well as being the diminutive form of the name "Meredith". The former is not a drunk, but (probably) some people with the latter name are.
And no-one who is sober and abstinent from liquor is going to get "merry" that way, so we should be able to say that to them, right?