Why today? :) - Because the lyrics of 'September' they were singing in Trolls movie credits were as follows: 'Do you remember the 21rst night of September... ' :) 

And I found THIS Online some time ago ⤵

- So I am Sharing it To Let ALL THE BROPPY FANS Celebrate! 😁😤👍

( & from now on Remember this Date!  ^_~ )