(Days of the week are always capitalized, so it's always Friday, not friday.) "By Friday" includes Friday. "Before Friday" does not.

Does by Friday mean end of Friday?

For many of us, by Friday means before the end of Friday.

What does it mean when you say by Friday?

by friday means the person saying it would want something to be completed on or before friday. example: a teacher wants a report turned in by friday. so you would have to finish it and turn it in before then.