This is Bissextile day. It has nothing to do with cross-dressing, or any other aberration.
Now, we all know that the earth’s orbit around the sun – the solar year – takes a little more time than 365 days, and if that time is not accounted for, eventually all those extra minutes add up. With enough additional time, the civil calendars no longer correspond to the solar year nor correctly pinpoint the seasonal markers, so that (for instance) the Vernal Equinox, the beginning of spring, might be off by three or four months.
[Small digression: remember that the Roman dates were calculated according to the Kalends, Ides, and Nones of the month, and were named as such, so that the sixth day prior to the first of the following month (inclusively) would be written ante diem sextum Kalendas _____________, and would correspond to our 26th or 27th of a month (those with 30 or 31 days) or as with February, the 24th.]
To continue: The Roman calendar was originally based on the phases of the moon, and in order to correct the difference between lunar and solar year, they inserted an intercalary month of 22 days between the 5th and 6th days before the Kalends of March (aka the 23rd and 24th of February) every once in a while. When this was done was determined by the pontifices – men with both religious and political authority to regulate Roman life – and would have worked, with a few refinements, had it been carried out systematically, but alas! It was in the hands of the politicians, and you know how they are. Sometimes, in the press of business, they forgot. Because the intercalation was considered unlucky, they might forego it during times of stress (like wars). Sometimes, in order to give themselves more time in a political office or shorten the term of a rival, they added more than one intercalary month in the year or deliberately left it out altogether.
[I shudder to think what a standing president would do with such power. Oh wait…]
By the time Julius Caesar arrived on the scene, the solar year and the calendar year were poles apart, which wouldn’t matter, I guess, until you realized that your Sextilis harvest festival is being held before your crops are even knee-high. So, among all his other accomplishments, Julius reformed the Roman calendar by first removing the intercalary month, then by permanently adding 10 days to the year, scattered among seven of the months, and one further day to February every four years. This day was entered where the old intercalary month had been – just before a. d. VI Kal. Mar., aka 24 February. It was known as ante diem bis sextum Kalendas Martias (the double-sixth day before 1 March), and the year in which it occurred as annus bissextus, eventually becoming, in English, bissextile.
[BTW, Julius was said to have learned of the idea of basing a civil calendar on a solar calendar from the Egyptians, with whose queen he was dallying.]
The two days, a. d. bis VI Kal. Mar. and a. d. VI Kal. Mar. were considered, for legal purposes, to be one day, but which was the day and which was the bis is open to debate. Thankfully, we don’t do this anymore, otherwise you would be working (by our civil calendar) one extra day this week for the usual amount on your paycheck. Boo! Hiss! Once the days were numbered consecutively in a month, the old Roman method of counting fell out of favor, and Leap Day is now the 29th of February.
Meanwhile, dear old Saint Matthias’s feast day was celebrated on a. d. VI Kal. Mar., but since this is a double day, the actual celebration was moved to tomorrow, the 25th, in Leap or Bissextile Years.
In the new General Calendar, his feast has been moved to May 14, making everything much simpler.
You can read more about the Roman calendar here at the Encyclopaedia Romana.