What is the origin of this tradition?

Explanation

Christmas stamps are issued around the world intended for use on seasonal mail such as Christmas cards. These stamps are regular postage stamps, unlike Christmas seals. It is a matter of some debate as to which is the first Christmas stamp. The Canadian map stamp of 1898 bears an inscription "XMAS 1898", but it was actually issued to mark the inauguration of the Imperial Penny Postage rate. But Xmas is Xmas!