Sounds like someone has a case of the Mondays.

no one says "happy Monday"

The proper response is just "Monday."

This is me but when people say "tgif" or post those "you made it to Friday" memes.

Monday is my Friday and Friday marks the beginning of the busy time of the week 🫩

At some point businessy people just started saying"Happy whatever-day-it-is". Happy Friday was the only acceptable one. 😅