There are fourteen possible calendars. How so, you ask? Take this notion that we have 52 weeks in a year. 52 x 7 = 364. That means a year is really 52 weeks plus one day.
That one day could be any of seven possible days: Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday. There’s no rule that says a year has to start on a particular day of the week!
If a year starts on a Sunday then the next year will start on a Monday, and the year after that on a Tuesday, then Wednesday, et cetera.
That’s where we get seven possible calendars. One for each day of the week. January 1st can fall on M, Tu, W, Th, F, Sa, or Su.
How do we get to fourteen? Easy: 7 x 2 = 14.
Not all years are 365 days. Some are 366 days. We call them Leap Years. The Leap Years add another wrinkle. We need a Leap Year version for each of our seven possible calendars. Thus there are fourteen.
You can re-use your calendars. Sounds weird, I know, but it’s true. Look up “Perpetual Calendar” in the almanac. Or go to this website.
2023 is classified as “Calendar 1.” This is arbitrary of course. But it makes sense as the year starts on Sunday and is not a Leap Year. Calendar 2 starts on Monday, Calendar 3 on Tuesday, you get the drift. Calendar 8 is, naturally, the Leap Year variant of Calendar 1.
Your old 2006 and 2017 calendars will work for 2023.
Your old 2022 calendar (Calendar 7) will work in 2033 and again in 2039.
The earth takes its own time traveling around the sun. It doesn’t care about our time-keeping schemes. We have to adjust to the physical reality. Plus there’s more than one way to measure a “year.” If you reckon a year by the stars, that is, you wait for the sun to return to the same place relative to its stellar background, that takes 365.2563 days give-or-take a few decimals. This is known as a sidereal year and is based on the idea of a fixed frame of reference, in this case the background constellations. The other way is to use geometry. If the sun moves 360 degrees—one complete revolution—relative to some starting point, that’s a tropical year. It’s about 356.2422 days from solstice to solstice. That’s 20 minutes shorter than the sidereal year. That extra 1/4 day (~0.25) or so is why they add a Leap Day every four years.
Myself, I count years from birthday to birthday. My year starts and ends on November 13th!
Happy New Year, everyone.