The song’s unstoppable groove made it a hit when it was released in 1978. More recently, annual Twitter videos made it a viral sensation.

Why the 21st? “We went through all the dates: ‘Do you remember the first, the second, the third, the fourth …’ and the one that just felt the best was the 21st,” Allee Willis, one of the song’s writers, told NPR. “There is no significance beyond it just sang better than any of the other dates.”

Oh, and what does “ba-dee-ya” really mean? It was a placeholder lyric that the band’s leader, Maurice White, opted to keep in the song: a lesson to Ms. Willis to “never let the lyric get in the way of the groove.”

That’s it for this briefing. See you next time. — Melina

Thank you

To Mark Josephson and Eleanor Stanford for the break from the news. You can reach the team at briefing@nytimes.com.

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