For religious reasons, when the Romans began to add days to some years to bring their calendar into line with the solar year, some time in the late 8th or early 7th century BCE according to legend, they created an extra month called Mercedonius to insert in their special leap years. They chose not to add Mercedonius after February, which was the final month of their year, but within it. February 24—known in the Roman calendar as “the sixth day before the Kalends of March” (a.d. VI Kal. Mart.) —was replaced by the first day of this month because it followed Terminalia, the festival of the Roman god of boundaries: https://www.bookofdaystales.com/boundary-day/ . After the end of Mercedonius, the rest of the days of February were observed and the new year began with the first day of March.
The Roman religious festivals of February were so complicated that Julius Caesar opted for a compromise to maintain the actual dates in February while still adding a leap day to the year one year in four in his 46 BCE calendar reform. The extra day of Julius Caesar’s leap year system was located in the same place as the old 1st of Mercedonius (after February 23rd) but he opted to ignore it as a date. Instead, the sixth day before the Kalends of March was simply said to last for 48 hours and all the other days continued to bear their original names. (The Roman practice of inclusive counting initially caused the priests in charge of the calendar to add the extra hours every three years instead of every four and Augustus was obliged to omit them for a span of decades until the system was back to where it should have been.) When the extra hours finally began to be reckoned as two separate days instead of a doubled sixth (“bissextile”) one, the leap day was still taken to be the one following directly on from the February 23 Terminalia.
Although February 29th has been popularly understood as the leap day of leap years since the beginning of sequential reckoning of the days of months in the late Middle Ages, in Britain and most other countries, no formal replacement of February 24 as the leap day of the Julian and Gregorian calendars has occurred. The exceptions include Sweden and Finland, who enacted legislation to move the day to February 29. This custom still has some effect around the world, for example with respect to name days in Hungary. Confused yet? Technically, in the Gregorian calendar, in leap years February 24th is the extra day, not the 29th. You are excused if you believe that this point is rather abstractly philosophical. Think of it this way. You have a line of 28 blue counters and you insert a red one in the line in the 24th position. You now have 29 counters, but the inserted one is the 24th and not the 29th.
Mercedonius or Mercedinus was also known as Interkalaris or Intercalaris. The leap year into which it was inserted was either 377 or 378 days long. It theoretically occurred every two (occasionally three) years, but was sometimes avoided or employed by the Roman pontiffs for political reasons regardless of the state of the solar year. This month, instituted according to Roman tradition by Numa Pompilius, was supposed to be inserted every two or three years to align the conventional 355-day Roman year with the solar year. The decision on whether to insert the intercalary month was made by the pontifex maximus (chief high priest), supposedly based on observations to ensure the best possible correspondence with the seasons. Unfortunately the pontifex maximus, who would normally be an active politician, often manipulated the decision to allow friends to stay in office longer or force enemies out early. Such unpredictable intercalation meant that dates following the month of Februarius could not be known in advance, and, in addition to this, Roman citizens living outside Rome would often not know the current date.
The exact mechanism for when to insert Mercedonius, and how long it was, is not clearly specified in ancient sources. I do like to focus on calendars other than our Gregorian calendar once in a while, because ours is generally so regular and predictable (and is corrected every so often by leap seconds to keep it synchronized with the solar year to a degree that ordinary people have no need for). Not to mention the fact that the Gregorian calendar has completely swamped all other calendars in the world, although they still show up – mostly for religious purposes. Easter, and allied celebrations from Lent to Pentecost, does give us one shot at being a little bit fast and loose with dates, but most of our celebrations are a little too routine for my tastes. An orderly calendar is comforting, but I am not unfailingly committed to order.
To help create some uncertainty here is an ancient Roman recipe for fried veal from Apicius:
Vitella fricta: piper, ligusticum, apii semen, cuminum, origanum, cepam siccam, uvam passam, mel, acetum, vinum, liquamen, oleum, defritum.
Fried veal: pepper, lovage, celery seed, cumin, oregano, dried onion, raisins, honey, vinegar, wine, liquamen, oil, defrutum.
Ancient Romans ate with their fingers without knives at the table, so after frying your veal you should cut it in strips, and serve it with the sauce. The ingredients for the sauce are straightforward except for liquamen and defrutum. I use Asian fish sauce for liquamen, which was a salty, fermented fish sauce. Defrutum was made by mixing red wine and fresh fruit (often figs), and boiling until the liquid was reduced by a half, straining and bottling. It is a syrupy sauce.
If you mix all of the sauce ingredients together in proportions of you choosing and then marinate the veal in the sauce before frying, you will have a complex dish. It should have a balance of sour, sweet, and salty along with the complex herb and spice mix.