Formal:
* Le vendredi - This is the most formal way to say "on Fridays." It literally translates to "the Friday."
Informal:
* Tous les vendredis - This translates to "every Friday" and is a more informal way to say "on Fridays."
* Le vendredi (without the definite article "le") - This is also informal, and it can be used when the context makes it clear you are talking about Fridays.
Example sentences:
* Le vendredi, je vais au cinéma. (Formal) - On Fridays, I go to the cinema.
* Tous les vendredis, on a cours de français. (Informal) - We have French class every Friday.
* Vendredi, je travaille de 9h à 17h. (Informal) - I work from 9am to 5pm on Friday.
The best way to choose which phrase to use depends on the situation and your desired level of formality.