No one knows what the crowd will be like on a specific day.
However, there are some basic principles which can be used to pick the best day for your visit.
This is the most accurate answer you're going to get to the "how crowded" question, without being the Director of Park Operations.
1. Start by checking the Cedar Point operating schedule. If the park closes at 8:00 p.m., it isn't because the midway is jammed with people. Fewer hours of operation means lower crowd levels.
2. The busiest time of the year is from July 4 through mid-August. Notice the longer hours of operation during this time.
3. Basically, all things being equal (i.e. dry weather), here is a breakdown of the busiest days, in order: Saturday, Wednesday/Monday/Friday/Thursday/Tuesday/Sunday. There are exceptions to this "rule" if it rains. If it rains on one day, the next dry day will see a bump in attendance.
4. This is a "tip" for people who plan a day trip and are trying to find a good time to go. Watch the weather forecast in the morning for Detroit and Cleveland. If it is raining and looks like it won't be an all day rain, go to the PARK. Don't waste time get there. Cedar Point will be "empty" that day. The two key markets for Cedar Point, Detroit and Cleveland, both being just a few hours or less away, are directly affected by the early morning weather in each city.
Hopefully you can make sense of this information and make and educated guess on the crowd level. Even if you go to Cedar Point during a busy time, the park is staffed to handle the crowd levels throughout that period. There have been a few blips to this "system" over the past few years. Closing Day in from a few years ago comes to mind. It was an incredibly nice day in October and everyone seemed to have the same idea of one last coaster ride. That just further proves that, even with the tips above, sometimes you just don't know what will happen. But, for the most part, this will help you.
Most of all remember: ANY DAY AT CEDAR POINT IS BETTER THAN BEING AT WORK.
(Inevitably, people want to know "how was it last year on <insert day here>." If you really must know, check out our extensive Trip Reports forum. You'll find reports and stories on past experiences for all times of the season. Just keep in mind that past experiences may not equal future ones!)