When did 'christmas' become 'xmas'?

Strictly spoken, never. "Christmas" is still more commonly used, especially in spoken language.

The use of "X" instead of "Christ" in this context dates back to at least 1551 as "X'temmas" and the exact "Xmas" is documented back to 1721.

The use of the X is not meant as a secularisation, but rather the letter does stand for the Greek chi (written as X) which, in turn, stands for Christ.

https://en.wikipedia.org/wiki/Xmas

The abbreviation "Xmas" has been around for quite some time. It originated as a handwriting abbreviation for "Christmas" and dates back to the mid-1500s. The "X" in "Xmas" comes from the Greek letter Chi, which is the first letter of the Greek word "Xpiotoc" (Christós), meaning "Christ". The suffix "-mas" comes from the Old English word for Mass.