Yes, well, you're wrong.
Red letter days are older than Christianity. There's a calendar dating from the Republic of Rome that uses red for important dates (my source, Wikipedia, says this calendar was used at some time between 509 and 27 BC, which is a bit odd because, let's face it, how hard is it to date a calendar accurately?).
The term red letter day seems to be to do with the church marking religious festivals in red ink, but of course that's a different thing.
In England red letter days now include both the Queen's birthdays (biological and official) and the anniversary of Her Majesty's coronation. These extra dates give High Court judges the chance to wear their best bright robes. Universities, not to be left out, have scarlet days when the academics get their chance to dress up in their brightest colours.
Nowadays, the idea of a celebration being a red letter day, or a red day, has spread as far as Hong Kong and South America.
Now, you will be thinking, this is all very well, but what are we to celebrate?
Well, how about The Word Den's three thousandth post?
I shall be raising a glass to all the many many readers who, over the years, have come to visit.
Thing To Have Today: a red letter day. The word red goes right back to the Sanskrit rohita, and probably beyond. Red is always (as far as was known last time I checked) the first colour, after black and white, to be given a name in a language.