Seals became popular early in the 20th century and have been issued on a yearly basis by many countries since that time. Christmas seals from the American Red Cross/American Lung Association are the most well known in the US but many others exist from around the world. These were used to solicit funds to help many charities.
The first stamp with a Christmas connection, although not targeted for use on Christmas mail, was the 1898 Canada stamp that had the word Xmas and the date of issue printed on it. The first stamp issued for Christmas mailings in Canada did not come until 1964. The first US Christmas stamp was issued in 1962. The pre-1965 issues are relatively small in number. Christmas V-mail and airgraphs were popular during WWII with a wide variety of Christmas motifs. Christmas-theme pictorial cancels were used by many countries many years before Christmas stamps were available.