A Palindrome Day happens when the day’s date can be read the same way backward and forwards. The dates are similar to word palindromes in that they are symmetrical.
Because date formats vary from country to country, not all dates that are considered palindromic in one kind of date format are Palindrome Days in another. For instance, September 10, 2019 or 9-10-19 is only a palindromic date in the m-dd-yy format, but it isn't if you write the date as mm-dd-yyyy (09-10-2019); as dd-mm-yyyy (10-09-2019) or as yyyy-mm-dd (2019-09-10).
Last Palindrome Week This Century
In 2019, every day for 10 consecutive days is a palindrome in the m-dd-yy format, which is common in the Unites States:
In addition to the format, many people around the world also write the digits of the date differently. For instance, it is common to write only the last two digits of the year – so, 19 instead of 2019. Other date representations include:
- d-m-yyyy or 10-9-2019
- d-m-yy or 10-9-19
- dd-md-yy or 10-09-19
- mm-dd-yy or 09-10-19
Each of these date formats has its own Palindrome Days.
Some Palindrome Dates are Rare
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.
According to Dr. Inan, in the mm-dd-yyyy format, the first Palindrome Day in the current millennium (January 1, 2001 to December 31, 3000) was October 2, 2001 (10-02-2001) and the last such day will be September 22, 2290 (09-22-2290).
There are 12 Palindrome Days in the 21st century in the mm-dd-yyyy format. The first one was on October 2, 2001 (10-02-2001) and the last one will be on September 2, 2090 (09-02-2090).
In the dd-mm-yyyy format, there are 29 Palindrome Days in the current century. The first was 10 February 2001 (10-02-2001). The last is a special one – it's a leap day! 29 February 2092 (29-02-2092) will be the last Palindrome Day of the 21st century.
The back-to-back Palindrome Days in the m-dd-yy format this week are not that rare. Every year since 2011 have had 10 consecutive Palindrome Days. In 2011, they occurred from January 10, 2011 (1-10-11) to January 19, 2011 (1-19-11). In 2012, the same sequence of dates occurred in February. In 2017, it happened in July, in 2018 in August and in 2019 it will happen in September.
Notice a pattern here? 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.
The month this happens corresponds to the last digit of the year. For example, in 1918, 2018 and 2118, the 8th month of the year, August, will have 10 Palindrome Days. Each of these days will begin on the 11th day of the month and end on the 19th.