What is the best day to visit Cedar Point?

Any Day it is not raining........oh wait, never mind.

Truthfully I find Tuesdays to be the best, but that is just me.

Sunday nights, between the weekend crowd and those who are taking vacation during the week, always seem nice.

But:

Any day I'm at an amusement park is a good day.

Today is. i rode maverick, mf, dragster and raptor all by 11:30

Sunday nights, between the weekend crowd and those who are taking vacation during the week, always seem nice.

But:

Any day I'm at an amusement park is a good day.

I can concur that information, based upon a visit several years ago. The Point was less busy Sunday afternoon/evening than it was on Monday. Not perfectly true, but seems to be common. But any day you can make the most of a visit, will be a good day to visit Cedar Point. Even on an extremely busy HalloWeekends Saturday I managed to have a great time (ie I did Gemini four times in two hours).

is it slower on Monday or Friday?

That all depends. I have found Sundays to be the least busy, especially during Halloweekends....