The x in Xmas represents the Greek letter chi, which is the initial letter of Christos - the Greek for Christ.
Christians came to use X as a symbol, and it crept into the English language as a shorthand. From there it became more generally used.
Xian for Christian and Xmas for Christmas are customary shorthand when religious writers are writing articles and books, and it gets 'corrected' by the tyepsetters.
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pi is not an abbreviation. It is a letter of the Greek alphabet symbol that symbolises one particular ratio.
The Greek alphabet.
Now there is no 27th letter in the alphabet but a very long time ago there was a letter called ampersand. It was the 27 letter of the alphabet. +++ The ampersand (originally "Amper's 'And' "after the publisher who invented it) still exists and thrives! It is not normally now regarded as a letter as such, but as it was conceived, an abbreviation for "and". It is used not only visually in text, sign-writing etc. but also as a common control-code in computer-programming.
Xmas
I've only ever come across Fri.