I happened to notice in an Old Farmer's Almanac once that St. Thomas the Apostle's Feast Day was listed as December 21. The "old" almanac must use an "old" saint resource ... for his feast day is now July 3.
However, in more recent saint books and in the most current Roman Martyrology, you will find St. Thomas listed as a July 3 saint. I'm not sure when the date change happened, or why. Perhaps it was among the many Vatican II changes; perhaps it was due to December 21 having such a focus on Advent that Thomas got overshadowed. Who knows ... feast dates do get changed now and then (Pope St. Gregory the Great's for another example) ... as long as St. Thomas is honored at some point, all is well!