1. Vivien and Fiona visit their friends on every Friday.

Question

2. Vivien and Fiona visit their friends every Friday.

3. Vivien and Fiona visit their friends on Fridays.

4. Vivien and Fiona visit their friends on Friday.

(Are all grammatical? Is Sentence 1 right? Thank you for your help.)

Answer 1

2, 3, and 4 are fine. 1 is not quite right -- no need for the word "on" or the word "every." One of them needs to be removed.