July and August (the first 2-3 weeks) are always the most crowded times at Cedar Point. If you have the option, wait until the end of August after schools have begun, or else go during those last few weekends when the park is still open. As for individual days, mid-week is best and Saturdays are generally the worst, but any single day really is a guessing game. This time of the season, hope for the best!
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Po!nt of View: A different look at Roller Coasters.
http://www.crosswinds.net/~justmayntz/thrills/index.html