R Dovid Soloveitchik Shlita said in the name of the Brisker
Dayan, R. Simcha Zelig Riger zt”l, “ Chazal said Choson Beyom
Chuposo Mochlin Lo Avonosov.,this is only on the same day of the Chupah.
If the Chupah is in the evening, then Yom Chuposo is not on the day
before the Chupah, and there is no need to fast.