just a guess, but i think it means that the day 17 has something to do with getting the secret on day 18 or 19 . it would be not the first time in the game that for a special thing to happen you need to do something days before.
the annoying thing for me is, since i dont know WHAT the secret is, i wont be able to tell if i got it or not XD
Viewing post in Version 0.67 (Haruki) - Public Release comments
the secret is a bug a tester triggered that turned Haruki naked. People liked it and Basket decided to keep the bug in the game, turning it into a secre, however, we don't know how to trigger the bug and Bask said it's a very specific process.
https://twitter.com/BaskyCase/status/1762938161968115822