For as long as I can remember Lego has been making Special Christmas themed sets every year. Advent calendars, Winter village sets, Christmas trains, and on and on.
What was the first Christmas themed Lego set?
I don't know whether this quite qualifies as a set, but there is a LEGO promotional Christmas box from 1964 listed on bricklink
There have been Christmas-themed sets for a long time, for example set 246-2 Santa and Sleigh from 1977.
Scrolling through all LEGO sets on Brickset, starting from 1949, shows that this is also the first set in their records that is Christmas themed, together with the (likely unreleased) two Santas set 245-2.