Formal:
* À mercredi! (Ah mer-creh-dee!) - This is the most common and direct way to say it.
* On se voit mercredi. (On seh vwah mer-creh-dee.) - This is a bit more formal, but still polite.
Informal:
* On se retrouve mercredi. (On seh reh-troov mer-creh-dee.) - This means "We'll meet on Wednesday."
* Mercredi, on se voit. (Mer-creh-dee, on seh vwah.) - This is a little more casual, but still polite.
Very informal:
* Mercredi, cool? (Mer-creh-dee, cool?) - This is very casual and only suitable for friends or close acquaintances.
It's important to choose the right phrase depending on who you're talking to and the context of the conversation.