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Earth, Wind, & Fire is celebrating the band's special day.

They tweeted a message to fans on Tuesday, Sept. 21 – a date made famous in their iconic song "September."

"Happy 21st of September!!" the band's official account wrote. "It’s officially Earth, Wind & Fire Day! Reply with a dance gif to keep the celebration going."

The song became the group's biggest hit when it was released in 1978 and features a question that has since then become rather iconic: "Do you remember the 21st night of September?"

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The lyrics of the song allude to the date being a fun one between two lovers, but the exact significance is never revealed.

As it turns out, that's because the significance is a bit murky even to the songwriters. Co-writer Allee Willis spoke to Songfacts back in 2008 about working with the band's frontman Maurice White. 

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The band was a longtime favorite of hers, so Willis recalled how she immediately jumped on board when White called her to join them for a writing session.

American singer-songwriter and musician Maurice White of American multi-genre band Earth, Wind & Fire performing, US, 3rd February 1978.

American singer-songwriter and musician Maurice White of American multi-genre band Earth, Wind & Fire performing, US, 3rd February 1978. (Photo by Michael Putland/Getty Images         )

"Just as I opened the door and I heard that little guitar intro, I thought, 'Oh God, please let this be what they want to work with me on,'" Willis recalled. "Because it was so obviously a hit."

The song had a loose concept of what became the chorus.

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"The, kind of, go-to phrase that Maurice used in every song he wrote was 'ba-dee-ya,'" Willis told NPR in 2014. "So right from the beginning he was singing, 'Ba-dee-ya, say, do you remember / Ba-dee-ya, dancing in September.' And I said, 'We are going to change 'ba-dee-ya' to real words, right?"

Needless to say, the lyrics never changed, despite Willis' desire to do so.

"And finally, when it was so obvious that [White] was not going to do it, I just said, 'What the f--k does 'ba-dee-ya' mean?' And he essentially said, 'Who the f--k cares?'" said the songwriter. "I learned my greatest lesson ever in songwriting from him, which was never let the lyric get in the way of the groove."

The famous opening line came together in a similar manner.

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"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," she shared. "I constantly have people coming up to me and they get so excited to know what the significance was. And there is no significance beyond it just sang better than any of the other dates. So ... sorry!"

Fox News' Nate Day contributed to this report.