February 2021 isn’t that special, and doesn’t happen once every 823 years

“This coming February will never come again in your life,” reads a message widely shared on Facebook. It claims this is because February 2021 will have four of each day of the week (four Mondays, four Tuesdays, and so on), something that only “happens once every 823 years”.

February 2021 will be a little special, but the message is wrong about why – and how rare this is.

Roughly three in every four Februaries will have 28 days

Anyone who spends time considering the calendar will quickly realise why the message’s main claim is wrong. Like most Februaries, February 2021 will have 28 days – exactly four weeks.

So the month will naturally have four of each day of the week. February 2022 will also have 28 days, and so four weeks and four of each day of the week.

The message seems to be a variation on a common myth about the calendar. The details differ but, for some reason, the myth always includes the false claim that some calendar quirk occurs “once every 823 years”. Variations of the claim have been made about August and October 2010, July 2011 and 2012, October and August 2014, and August 2020.

So how is February 2021 special?

Leap years and Monday the first

To begin with, some Februaries do not have four of each week day. Certain years will have leap days, a 29th day added to February to account for the fact that a year (one orbit of the earth around the sun) is slightly longer than 365 days. This happens every four years, except in years divisible by 100.

This is because ordinary leap years overcorrect for the rotation of the earth around the sun and are off by one day every 100 years. As celebrity astrophysicist Neil deGrasse Tyson explains, years that are exactly divisible by 100 are not leap years. And to prevent overcorrection again, every year exactly divisible by 400 will include a leap day.

So the year 1900 was not a leap year, and 2100 won’t be one. But 2000 was a leap year. But not falling in a leap year still doesn’t make February 2021 particularly special. So what does?

The first day of the month, 1 February 2021, is a Monday. This means that the beginning of the week and the beginning of the month fall on the same day. And because it isn’t a leap year, the last day of the month will fall on a Sunday, the last day of the week.

Of course, this isn’t unusual either. It was the case in 2010, and will be the case again in 2027. This is even true of the year 2100, because it is divisible by 100 (but not 400) and will not be a leap year.