It's when you're dealing in an official capacity or speaking to a broad audience or when you don't know the person's culture.
The CEO saying Merry Christmas to his 140 employees, when 5 of them are Jewish is going to be not feel so great for those 5 Jewish people. Happy Holidays should be fine for everyone.
But if you know the person is Christian (or celebrates Christmas) it should be perfectly fine to tell them Merry Christmas.
Of course in some places that may be considered insensitive because a Jewish person might be hearing it. Which is absurd and that level of sensitivity is not acceptable IMO.
It's a monday. So that's already more like 1 in 52. There's been like 5-20 news worthy "return to work" announcements in the past year, I'm guessing half othem have mandatory 2 days, the other half have mandatory 3 days.
Multiply that by the number of things that happen in your life where a coincidence of this level could happen and you should be seeing this kind of coincidence a many times each year.