5 Dates You Won't Find on Your Calendar

While you may be a bit fuzzy on which months have 30 days and which have 31, it’s safe to say you’re pretty familiar with the months and days of the year. But due to unusual record-keeping practices or because of months that were eliminated over the centuries, there are a few days you’ll never see on your joke-a-day calendar.

1. JANUARY 0

At midnight every New Year’s Eve, we go from December 31 to January 1. Simple, right? That’s how it works for everyone ... except astronomers. Each year, astronomers keep track of the movements of various planets and stars, which are compiled into what’s called an ephemeris. While it’s useful for things like space travel and positioning telescopes, GPS systems also use the data to properly function.

The thing about ephemerides (the plural of ephemeris) is that they don’t reference any year other than the one for which they were written. So if you had an ephemeris for the year 2000, you wouldn’t find any mention of 1999 or 2001. Generally speaking, it shouldn’t be necessary, though, since it’s only for that particular year anyway.

Except when you referenced January 1, that is. Because some more detailed ephemerides will list the previous day’s celestial positions for reference purposes, the ephemeris would have to have information for December 31. But, since the ephemeris doesn’t refer to any other year, this date will instead often be called January 0. Going back to our year 2000 example, an ephemeris for that year might list Prince’s favorite day, December 31, 1999, as January 0, 2000 instead.

It’s worth noting that many modern day ephemerides have dropped the use of January 0 entirely, but there are others that still use it.

And back in the 1920s, several groups lobbied for a calendar with 13 months, each with four weeks. To reach 365 days, their plan was to add "January 0."

2. FEBRUARY 30

You may have a friend or relative in your life whose birthday is February 29. Maybe they fudge it and celebrate on February 28 or March 1 every year, or possibly they just have a mega-party every four years. (Or they have a mega-party every year, because why not.) So imagine how frustrating it’d be to have been born in the Swedish Empire on February 30, 1712, the only day of its kind in history.

Naturally, it was a pretty complex set of events that led to February 1712 getting two leap days. Our modern, Western calendar is called the Gregorian calendar, which was developed under Pope Gregory XIII. It’s basically just a series of improvements to the Julian calendar, created by Julius Caesar.

While the Gregorian calendar was completed in 1582, adoption by many countries was slow, so it took over 100 years for the Swedish Empire (which was primarily Protestant and not Catholic) to adopt it. Because the Julian to Gregorian swap included a difference of ten days, many regions simply skipped their calendar ahead a week and a half. The Swedish Empire decided to roll out the difference more gradually, and intended to skip leap days for forty years, starting in 1700, until the calendar was finally correct.

Except that didn’t happen because, shortly afterward, war broke out and everyone forgot about the leap days until 1712, when Sweden’s King, Charles (or Karl) XII, declared that they would forget about the Gregorian calendar and just switch back to the Julian instead. Since they did manage to skip one leap day, in 1700 (which was a leap year under the Julian calendar, but not the Gregorian), they simply decided to add it back onto the calendar that February—meaning that February 1712 had two leap days according to Sweden’s calendar, which gave them the only February 30 in history. (Sweden finally went through with the Gregorian switch in 1753 and just skipped ahead a few days, like everyone else.)

3. MARCH 0

While you could think of February 30 as some weird kind of March 0, they’re not the same thing (though they do both involve leap years). If someone asked you what the day before March 1 is, you’d probably ask them, “What year?” March 0 is, like January 0, simply a reference to the day before it, but it’s useful since March 0 can be either February 28 or 29, depending on the year.

While this is occasionally used in software (some old versions of Microsoft Excel will accept 3/0 as a date and simply plug in the correct day for the particular year, for example), it’s more commonly found in something known as the Doomsday rule.

It sounds fairly ominous, but the Doomsday rule is just a method for calculating what day of the year falls on for any given date. For example, by following the Doomsday rule, you could quickly tell that January 19, 1481 was a Wednesday. How? By figuring out what creator John Conway calls “the Doomsday.” This is the day of the week that certain calendar days will always fall on in a given year. April 4, June 6, and August 8 are just a few days of the year that will always fall on that year’s Doomsday. Another big one? March 0, i.e., the final day of February.

So, using 1481 as our example again, you can use a formula to determine that its Doomsday was Monday. (For the record, 2013’s Doomsday is Thursday.) From there, we could quickly ascertain that March 0 was a Monday, and for that particular year, February only had 28 days (since it was not a leap year), making “March 0” Monday, February 28, 1481. If you’re mathematically-minded, it’s a fun challenge. If you’re not, well, you can always look the day up on the internet or use a Doomsday calculator.

4. UNDECIMBER AND DUODECIMBER

There aren’t just odd and unusual days of the year. There are entire months as well. Remember the episode of The Simpsons where the school ordered faulty calendars with a 13th month (called Smarch)? Well, as it happens, we kind of had that once upon a time—namely, those left over from the days of the Roman calendar, which preceded the aforementioned Julian calendar. Much like how the process of moving from the Julian calendar to the Gregorian calendar left a few odd days out, the move from the Roman calendar to the Julian one actually added some.

These days, 67 in all, were then added into a pair of months between the November and December of 45 BC, and were referred to as intercalaris prior and intercalaris posterior, which are often called Undecimber (pronounced like “oon,” not “uhn”) and Duodecimber in modern days.

These names refer to the fact that December is named after the Latin word for ten (which itself came from the fact that the Roman calendar originally only had ten months and not twelve), while the Latin words for eleven and twelve (or in this case, thirteen and fourteen) are undecim and duodecim.

What’s more, the terms have even come to be used in modern computing. The Java programming language includes support for a 13-month calendar, and it refers to the 13th month as Undecimber.

5. MERCEDONIUS

Speaking of the Roman calendar, by the time Julius Caesar came along, it hadn’t had ten months for quite a while. Nearly 600 years, in fact. The Roman calendar that Caesar reformed was itself a reformed calendar constructed by King (not Emperor) Numa Pompilius sometime in the 7th century BC.

Prior to Pompilius’ changes, the Roman calendar, as we mentioned, had ten months: Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November, and December. (Quintilis was later renamed Julius after Julius Caesar himself, while Sextilis was changed to Augustus in honor of his son/grand-nephew, Caesar Augustus.) King Numa Pompilius added Januarius and Februarius, giving us the twelve months we have today … except he also added another, forgotten month that hasn’t been in use for millennia: Mercedonius.

Mercedonius was a kind of a leap month, situated between Februarius and Martius, and was approximately 27 days. Although there was apparently some kind of formula to determine in which years Mercedonius was used and in which years it wasn’t, the implementation was spotty, since it was up to whoever the current Pontifex Maximus was at the time to decide if the month was used or not.

Since the month was used so sloppily, Julius Caesar simply eliminated it entirely when constructing the Julian calendar, rearranged the days throughout the year, and made a simple, easy-to-follow leap day system.