A Simple Trick For Figuring Out the Day of the Week For Any Given Date

Tevarak/iStock via Getty Images Plus
Tevarak/iStock via Getty Images Plus

People typically remember anniversaries in terms of dates and years, not days of the week. If you can’t remember whether you got married on a Saturday or Sunday, or don't know which day of the week you were born on, there’s a simple arithmetic-based math trick to help you figure out sans calendar, according to It's Okay To Be Smart host Joe Hanson.

Mathematician John Conway invented the so-called Doomsday Algorithm to calculate the day of the week for any date in history. It hinges on several sets of rules, including that a handful of certain dates always share the same day of the week, no matter what year it is. (Example: April 4, June 6, August 8, October 10, December 12, and the last day of February all fall on a Wednesday in 2018.) Using this day—called an “anchor day”—among other instructions, you can figure out, step by step, the very day of the week you’re searching for.

Learn more about the Doomsday Algorithm in the video below (and if it’s still stumping you, check out It’s OK to Be Smart’s handy cheat sheet here).

20 Weird Clubs That Actually Exist

Mental Floss via YouTube
Mental Floss via YouTube

Groucho Marx once famously quipped that he'd never "want to belong to any club that would accept me as one of its members." Most people would probably say the same about the Martin-Baker Ejection Tie Club—a very exclusive, 63-year-old organization created specifically for individuals who have had their lives saved by an ejection seat. Currently, the club boasts more than 6000 members.

That's just one of the weird and wonderful clubs you'll learn about in our latest edition of The List Show. Join Mental Floss editor-in-chief Erin McCarthy as she hunts down the world's most unusual clubs (Extreme Ironing Bureau anyone?). You can watch the full video below.

For more episodes like this one, be sure to subscribe here!

Video Captures Fiery Eruption of Mexico's Popocatépetl Volcano

RobertoVaca, iStock via Getty Images
RobertoVaca, iStock via Getty Images

Mexico is home to 48 active volcanoes, but few can compete with Popocatépetl. Located around 40 miles southeast of Mexico City, it's one of the most active volcanoes in the country, and on January 9, the extent of its power was caught on camera.

The video above, reported by NPR, shows the Popocatépetl stratovolcano—also known as a composite volcano—spewing lava, ash, and rock in a fiery plume that reached 20,000 feet above its cinder cone crater. CENAPRED, Mexico's National Center for Disaster Prevention, filmed the volcanic eruption as it unfolded early Thursday morning. The National Oceanic and Atmospheric Administration also recorded the explosion from space using its GOES 16 satellite.

No one was hurt by the incident last week, but CENAPRED is warning people to avoid the area as debris continues to fall from the summit. The center has set its Volcanic Warning Light to Yellow Phase 2, which indicates there's no immediate threat of danger.

Since it emerged from dormancy in 1994, Popocatépetl, or "El Popo," as it's known by locals, has become one of the most active volcanoes in Mexico. Tremors and showers of ash are now regular occurrences for residents of nearby towns. Given its volatility, there are currently 20 devices monitoring the volcano 24/7.

[h/t NPR]

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