Happy Friday 13th, y’all

Is Friday 13th really unlucky?

Last year’s Friday 13th (in November 2020) happened exactly 39 (3 x 13) weeks before this year’s sole Friday the 13th on August 13, 2021. Next year’s sole Friday the 13th on May 13, 2022, will come exactly 39 (3 x 13) weeks after 2021’s Friday 13th. And that’s just the tip of the iceberg. We could cite many interesting correspondences between dates and the number 13. Are all these Friday the 13ths a super coincidence? Super unlucky? Neither. They’re just a quirk of our calendar.

Whenever a calendar month starts on a Sunday, that month will always have a Friday the 13th. In 2021, August 1 was a Sunday. And, sure enough, the 13th of this month is a Friday. It’s the only Friday the 13th in 2021. The last calendar year with just one Friday 13th was 2016 (in May). After this year’s solitary Friday the 13th, the next year with only one Friday 13th will be 2022 (in May).

This year’s solitary August Friday the 13th will recur in six years, on Friday, August 13, 2027.

Two Friday the 13ths last occurred in the leap year 2020 (March 13 and November 13, 2020). Two Friday the 13ths will next occur in the year 2023 (January 13 and October 13, 2023).

Three Friday the 13ths last happened in 2015 (February 13, March 13 and November 13, 2015). A triple header (with the same sequence) will repeat 11 years later: February 13, March 13 and November 13, 2026.

Friday 13th in folklore

According to folklorists, there’s no written evidence that Friday the 13th was considered unlucky before the 19th century. The earliest known documented reference in English appears to be in Henry Sutherland Edwards’ 1869 biography of Gioachino Rossini.

Still, Friday has always gotten a bad rap. In the Middle Ages, people would not marry – or set out on a journey – on a Friday.

There are also some links between Christianity and an ill association with either Fridays or the number 13. Jesus was said to be crucified on a Friday. Seating 13 people at a table was seen as bad luck because Judas Iscariot, the disciple who betrayed Jesus, is said to have been the 13th guest at the Last Supper.

Meanwhile, our word for Friday comes from Frigga, an ancient Scandinavian fertility and love goddess. Christians called Frigga a witch and Friday the witches’ Sabbath.

In modern times, the slasher-movie franchise Friday the 13th has helped keep friggatriskaidekaphobia alive.

Friday 13th number play

We have one Friday the 13th in 2021 (August) because 2021 is a common year of 365 days that starts on a Friday. Whenever a common year of 365 days starts on a Friday, it’s inevitable that the month of August will start on a Sunday. And, as mentioned before, any month starting on a Sunday will have a Friday the 13th.

The last time a common year started on a Friday was 11 years ago, in the year 2010. The next time will be six years from 2021, in 2027. Some of you may wonder if there’s some formula that governs how this Friday the 13th drama repeats itself. The answer is yes. Keep in mind that this sole August Friday the 13th year can only happen in a common year of 365 days, and when January 1 falls on a Friday.

Cycles of 6, 17 and 28 years

Any calendar year that happens one year after a leap year will recur in 6, 17 and 28 years. Therefore, if our solitary Friday the 13th year comes one year after a leap year, as it does in 2021, then the days and dates will match up again in 6, 17 and 28 years. Therefore, the years 2027, 2038 and 2049 will all feature a sole August Friday the 13th:

2021 + 6 = 2027

2021 + 17 = 2038

2021 + 28 = 2049

Within the confines of the 21st century (2001 to 2100), we can project this cycle onward:

2049 + 6 = 2055

2049 + 17 = 2066

2049 + 28 = 2077

Then again:

2077 + 6 = 2083

2077 + 17 = 2094

2077 + 28 = 2105 (No! This date is outside the 21st century)

August Friday the 13th in a leap year

By the way, an August Friday 13th can also happen in leap year of 366 days if that leap year begins on a Thursday. But it won’t be a solitary Friday the 13th. You’ll have a Friday, February the 13th, as well. This happened for the first time this century in 2004 (February 13 and August 13, 2004), and will recur 28 years thereafter for rest of the century (2032, 2060, 2088). In these leap years, Friday, February 13, comes exactly 26 (2 x 13) weeks before Friday, August 13.

Suppressed leap year perturbs cycle

You might think the year 2105 should present a sole Friday, August the 13th. However, by Gregorian calendar rules, the leap year is suppressed in the century years 2100, 2200 and 2300. Thereby the cycle is perturbed.

The years 2100, 2202 and 2309 present a sole Friday the 13th. Yet, the cycle of 6, 17 and 28 years works within the confines of a given century.

Because the Gregorian calendar has a 400-year cycle, the solitary August Friday the 13th years recur in cycles of 400 years.

As magical as all of this Friday the 13th calendar intrigue appears to be, it’s not supernatural. It’s just entertaining number play, of the sort that may haunt uncomprehending minds.

Bottom line: Any calendar month starting on a Sunday always has a Friday 13th. In 2021, August 1 is on a Sunday. August 13 must fall on a Friday!