Which day of the week gains the highest score in the game of scrabble

Technically, none of them. None of them are valid words in the TWL word list, being proper nouns.

However, if you were to score them, this is how they would stack up:

Wednesday: 17
Thursday: 15
Friday: 13
Monday: 12
Saturday: 12
Tuesday: 11
Sunday: 10

Additionally, if you played seven letters of Tuesday, Wednesday, Thursday, or Saturday, then their scores would be 50 points higher for the bingo.