The opening line of Earth, Wind & Fire’s “September” is one of the most famous lyrics of all time: “Do you remember the 21st night of September?”

While this is definitely one of those lyrics that has been analyzed for a deeper meaning many times throughout the years, the meaning of the date is actually quite simple.

In 1978, Allee Willis was a struggling songwriter, but she managed to form from strong friendships with really remarkable talents. Through big names like Patti Labelle and Herbie Hancock, Willis was introduced to Verdine White, the bassist of Earth, Wind & Fire. So, while they were prepping their greatest-hits compilation later that same year, Willis got a call from the legendary funk band.

“Is this Allee Willis?’” the songwriter recalled being asked over the phone. “This is Maurice White (the band‘s co-lead singer and primary songwriter). I want you to come write the next Earth, Wind & Fire album.” Of course, Willis was stoked. “I absolutely loved them. I always loved them, they were my favorite group,” she admitted to Songfacts years later.

When she arrived at the studio for her first day with the band, a catchy riff was already coming from the room.

“I opened the door and I heard that little guitar intro,” Willis explained. “I thought, ‘Oh, God, please let this be what they want to work with me on.’ Because it was so obviously a hit.”

Lo and behold, the “hit” she was hearing was an early stage of “September,” a new song the group planned to include on The Best of Earth, Wind & Fire, Vol. 1 in an effort to boost sales for the compilation. Upon hearing the instrumental, Willis immediately began crafting lyrics with the band, working to assemble an upbeat song that also contained a slight sense of nostalgia. Maurice White already had a concept for the chorus, though it didn’t have any real words yet.

“The kind of go-to phrase that Maurice used in every song he wrote was ‘ba-dee-ya,’ ” Willis explained in a 2014 interview with NPR. ”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?’”

As the song progressed, verses were written and laid out and the band prepared to record the new track. But still, the chorus hadn’t changed, and Willis began to realize it never would.

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

So, as they continued thinking about that opening phrase, the band tried out many other dates.

“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,” Willis explained. ”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!”

While the song is called, “September” it was actually released in November 1978. The song reached No. 1 on the Billboard Hot R&B Songs chart and No. 8 on the Billboard Hot 100, and easily became one of Earth, Wind & Fire’s most beloved songs ever.