Christmas is also sometimes known as Xmas. Some people don't think it's correct to call Christmas 'Xmas' as that takes the 'Christ' (Jesus) out of Christmas. (As Christmas comes from Christ-Mass, the Church service that celebrated the birth of Jesus.)
But that is not quite right! In the Greek language and alphabet, the letter that looks like an X is the Greek letter chi / Χ (pronounced 'kye' - it rhymes with 'eye') which is the first letter of the Greek word for Christ, Christos.
The early church used the first two letters of Christos in the Greek alphabet 'chi' and 'rho' to create a monogram (symbol) to represent the name of Jesus. This looks like an X with a small p on the top: ☧
The symbol of a fish is sometimes used by Christians (you might see a fish sticker on a car or someone wearing a little fish badge). This comes from the time when the first Christians had to meet in secret, as the Romans wanted to kill them (before Emperor Constantine became a Christian). Jesus had said that he wanted to make his followers 'Fishers of Men', so people started to use that symbol.
When two Christians met, one person drew half a basic fish shape (often using their foot in the dust on the ground) and the other person drew the other half of the fish. The Greek word for fish is 'Ikthus' or 'Ichthys'. There are five Greek letters in the word. It can also make up a sentence of Christian beliefs 'Ie-sous Christos Theou Huios So-te-r' which in English means "Jesus Christ, Son of God, Saviour". The second letter of these five letter is X or Christos!
So Xmas can also mean Christmas; but it should also be pronounced 'Christmas' rather than 'ex-mas'!