The Feast of St. Thomas: December 21 or July 3?

14th Century Illumination of St. Thomas
 I happened to notice in our Old Farmer's Almanac that St. Thomas the Apostle's Feast Day is listed on December 21.  The "old" almanac must use an "old" saint resource ... for his feast day is now July 3.
The Old Farmer's Almanac - 2016

He's also listed on the December 21 page in an 1897 Roman Martyrology, and in other older sources or books set in earlier times (Catherine, Called Birdy for example).

Martyrologium Romanum - 2004
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!

The Incredulity of St Thomas
by Ludovico Mazzolino circa 1522