Contrary to popular belief, Jesus wasn't born December 25th, he was born on January 7th.
I don't think anyone actually knows the TRUE date of Jesus's birth, record keeping back then kinda sucked. But the rest of that stuff about pegan holidays and stuff is true.
Some may say Jan. 7th (I think Greek Orthodox say the 6th), the Roman Catholics say the 25th. But both sides have been saying it since the second century, so it's not as recent as many think. This debate is actually where we get the "twelve days of Christmas" stuff.
If one were to go by Biblical teaching, there wouldn't be any special holy days on the Christian calendar aside from those laid out in scripture (which don't include Christmas or Easter). But people like their holidays.
When did Squirrel become a devout Christian?
When has he not been a Christian? Do you even know who he is?
Heh, heh, heh, Bueno...

I've been a Christian since 1985. As for who I am, try visiting my
Brand New Hovel for a hint. Not that you would necessarily know me, but I'm a Spherian from way back.
As for the "X" I think it's supposed to represent a leaning cross. I guess lower-case "t"-mas doesn't sound good to anyone but Mr. T.
Dude, I'd celebrate that! Tmas!
"X" is the Greek letter ("chi") that comes first in the Greek spelling of Christ (see attached image for the actual Greek spelling). It's been used as an abbreviation for the word "Christ" for hundreds of years.