Let's say i have an assignment where the teacher writes that "this assignment is to be done at your leisure during the summer." does that mean that the assignment is optional?

I believe it is optional

I think that any assignment assigned during the summer is optional, because you're not in school during that time frame, however it may be in your best interest to do the assignment, for the well-being or improvement of your grades. It might be optional, but I would definitely recommend you do it.