The Reason for the Season
Santa is leader of the "Let's put the Ho back into ChristHOmas" campaign, and this is the origin of his catchphrase "HO, HO, HO". Exactly who Chris Thomas is is still debated by scholars and academics.
War on Christmas
The War on Christmas is real, and is a secret plan by Martians who have infiltrated all levels of Earth society as a complicated plan of revenge on Santa after their 1964 defeat (see Santa Claus Conquers the Martians for details.)
The True Meaning of X-mas
The X in X-mas comes from the Greek letter Chi, Christ's initial. It is a little known fact that that X is The X-Men is also short for The Christ-Men and the X in The X-Files is short for The Christ-Files.
The Evil Version of Santa
Although the Krampus (an evil creature of Alpine folklore who punishes bad children) was originally black, he is now exclusively white after Megyn Kelly declared it to be so.
The Evil Version of Satan
A lot is made of the similarities between Santa Claus and Lucifer. Santa is an anagram of Satan, and unsurprisingly Satan is also an anagram of Santa. Satan is often called Old Nick, and Santa is based on St Nick. Santa now almost exclusively wears red, Satan is often portrayed as being red in colour. There are also alleged connections to break and enter and an unhealthy interest in children. These, of course, are only coincidences and Satan has been a lot more careful with more recent disguises.
Santa Claus is often depicted as having poor workplace policies (the Elves being slaves or sweat-shop workers). However, he does have a strong anti-bullying stance. A reindeer called Olive was removed from his employ for laughing at Rudolph and calling him names.
Rudolph the Red-nosed Reindeer
Medical experts have suggested that Rudolph is suffering from an extreme form of Rosacea, a condition in which excessive consumption of alcohol causes the nose be become red, and often results in Rhinophyma (a bulbous nose). Rudolph is, in all likelihood, drunk when leading Santa's sleigh.
Yule, the pagan midwinter festival upon which Chi-mas is based is named after the internet spelling of "You'll".
The Wise Men
The Bible makes no statement about the number of Wise Men who visited the baby Christ. Three is the assumed number, however there were actually four. The one who brought gold, the one who brought frankincense, the one who brought myrrh and the one who complained bitterly he wasn't told that they were bringing gifts and could he chip in some money for the gold or something.
The Holiday Special
Although science fiction fans like to celebrate fictional events as real, there has been no movement by Star Wars fans to incorporate the Wookiee holiday "Life Day" into Christmas. This is not as commonly thought because The Star Wars Holiday Special was actually a Thanksgiving Special, not a Christmas Special, but in fact because The Star Wars Holiday Special really sucked.