On September 9 2019 and subsequently, a “palindrome week” meme circulated on Twitter:
Above 10 dates ranging from September 10 to 19 2019, text read:
Each date reads the same backwards and forwards for the next 10 days.
An attached status update stated:
HOW COOL ???? | It’s palindrome week. Each date is the same forwards and backwards!
A palindrome is a word, phrase, or string of letters or numbers that reads the same forwards and backwards. They Might Be Giants’ song “I Palindrome I” was devoted to, you guessed it, palindromes. In 2015, Grammarly explained:
According to The Oxford English Dictionary the word is based on Greek root words meaning “back” and “running.” Palindromes are words or phrases that read the same backward and forward, letter for letter, number for number, or word for word. Some palindromes seem philosophical. Do geese see God? Others tell a story. A man, a plan, a canal: Panama. Still others are silly and rather nonsensical. Straw? No, too stupid a fad; I put soot on warts.
Interest in palindrome week or palindrome dates in general wasn’t new in September 2019. On a palindrome day in February 2012, one site provided a list of all palindrome dates for the next 10 years (through 2022). However, that list only included September 10 2019.
In May 2015, LiveScience marked that year’s week with information about palindrome dates in general:
Every day this week — in fact, every day between May 10 and May 19, as written in five- or six-digit month/day/year format — is a palindrome. This means the numbers read the same forward as backward (5/10/15 – 5/19/15). And Mother’s Day was a seven-digit palindrome (5/10/2015), only the eighth such date that has occurred so far this century. Strings of five-digit palindrome dates aren’t actually that rare, according to Aziz Inan, an electrical engineering professor at the University of Portland, who calculates these dates as a hobby. “Whereas five-digit dates occur every year, the next time Mother’s Day will be a palindrome date will be 790 years” from now, Inan told Live Science. [The 9 Most Massive Numbers in Existence] The next such palindrome week will occur in June 2016 (6/10/16-6/19/16), and this pattern of five-digit palindromes will continue until 2020. After that, another type of five-digit palindrome will take its place, with two-digit months and one-digit days (for example, 02/1/20), [University of Portland professor Aziz Inan] explained.
In June 2016 (sixth month) and July 2017 (seventh month), there was one palindrome week each. In August 2018 (the eighth month), KVAL heralded yet another palindrome week — noting, however, that outside the United States, palindrome weeks came along in a different form:
But what about elsewhere in the world, where the date August 1, 2018, is expressed not as month-day-year – like 8-1-18 – but as day-month-year, like 1-8-18? They’ll have to wait until October for their next palindrome day. “In the rest of the world, 8/10/2018 will occur on 8 October 2018,” Inan said. “Also, they don’t have 11, they have only 4 palindrome dates this year instead of 11.”
Inan was frequently consulted when palindrome weeks or dates rolled around. TimeAndDate.com explained:
Depending on date formats, palindromic dates can be rare. Aziz S. Inan, an electrical engineering professor at the University of Portland, has calculated that in the mm-dd-yyyy format, Palindrome Days tend to occur only in the first few centuries of each millennium (1000 years). The last palindromic date in the second millennium (years 1001 to 2000) in this format was August 31, 1380 or 08-31-1380.
As long as you write your date in the m-dd-yy format, every century has 9 years with 10 Palindrome Days in a row. These years are always in the second decade of the century. For example, every year between 2011-2019, 2111-2119, and 2211-2219 will have 10 consecutive Palindrome Days. This is true for previous centuries as well.
According to Inan, the last palindrome date in the current millennium will be September 22, 2290. And September 2019 was the last palindrome week for the century.
In separate calculations on the University of Portland website [PDF] for “Palindrome Dates in the 21st Century,” no palindrome weeks appeared. That set of numbers included:
In the Month/Day/Year Date Format (26 seven-digit, 12 eight-digit, total 38 palindrome dates in the 21st century)[.]
In the Day/Month/Year Date Format (9 seven-digit, 29 eight-digit, total 38 palindrome dates in the 21st century)[.]
Those palindrome dates were calculated in plain strings, such as 091019 and 02022020 for February 20 2020. When thus formatted, September 12 2019 (09122019) and its following dates were not palindromes. In order to declare a palindrome week, numbers had to be formatted without the first zero (9 versus 09).
A palindrome week took place each year from January 2011 in January to September 2019, although the internet didn’t seem too interested in Palindrome Week 2011; interest seemed to really pick up around 2015. The first palindrome date in the same format after the final day of palindrome week 2019 was February 02 2020 (2022020.)