We use the word “on” before days of the week.
“I have to work on Monday.”
“She has to leave on Friday.”
If something happens every week, we can say something like, “I take English lessons on Wednesdays.” That means not just this Wednesday, but every Wednesday, week after week.
“She works part-time on Mondays, Wednesdays, and Fridays.”
We use “on” before specific dates when the month and numbered day on the calendar are mentioned.
“His birthday is on August 1st.”
“He was born on the 4th of July.”
“In both cases, on comes before the date.”