Formal:
* Ci vediamo venerdì. (This is the most common and neutral option.)
Informal:
* A venerdì! (This is more casual and friendly.)
* Ci becchiamo venerdì. (This is even more casual, and means something like "catch you Friday.")
You can also use:
* Ci sentiamo venerdì. (This means "We'll talk on Friday.")
* Ti aspetto venerdì. (This means "I'm waiting for you on Friday.")
Choose the expression that best suits the situation and your relationship with the person you're talking to.