Thursday the 20th original screenshot from the Simpson's episode, Pranksta Rap.

The "Thursday the 20th" meme comes from an episode of The Simpsons in which a rapper is performing on stage wearing a necklace with the words, "Thursday the 20th" on it. 

During the episode, Bart attends a rap concert without permission so Lisa and Principle Skinner use footage of the rapper's chain to prove that Bart actually attended the concert. 

The meme began spreading back in 2014 when a Reddit user posted a screenshot of the rapper to /r/TheSimpsons with the caption, "Happy Thursday the 20th, everybody!" Ever since then, anytime it's Thursday the 20th, this meme shows up in full force. 

In 2017, Thursday the 20th occurred on the same day as famous stoner holiday 4/20 so the image was photoshopped to make the rapper appear high. 

Thursday the 20th meme with red eyes for 4/20

Here's a clip from the episode:

Here's a list of all the days Thursday the 20th occur for the next 10 years so you can be prepared:

2020Thursday, February 20, 2020February+ 8 months2 times in year 2020
Thursday, August 20, 2020August+ 6 months 
2021Thursday, May 20, 2021May+ 9 months1 time in year 2021
2022Thursday, January 20, 2022January+ 8 months2 times in year 2022
Thursday, October 20, 2022October+ 9 months 
2023Thursday, April 20, 2023April+ 6 months2 times in year 2023
Thursday, July 20, 2023July+ 3 months 
2024Thursday, June 20, 2024June+ 11 months1 time in year 2024
2025Thursday, February 20, 2025February+ 8 months3 times in year 2025
Thursday, March 20, 2025March+ 1 months 
Thursday, November 20, 2025November+ 8 months 
2026Thursday, August 20, 2026August+ 9 months1 time in year 2026
2027Thursday, May 20, 2027May+ 9 months1 time in year 2027
2028Thursday, January 20, 2028January+ 8 months3 times in year 2028
Thursday, April 20, 2028April+ 3 months 
Thursday, July 20, 2028July+ 3 months 
2029Thursday, September 20, 2029September+ 14 months2 times in year 2029
Thursday, December 20, 2029December+ 3 months