No one here has a crystal ball, so use some common sense. Weekends will be more crowded than weekdays. Summer, especially when schools are out from mid-June to early September, are when crowds are at their peak.
Events, concerts, and school outtings also tend to be crowded. Special events, such as Halloween fright fests, will typically have large crowds. Most parks will show such events on their website but if you are unsure or want to CYA [cover your ass] CALL THE PARK!
Also, weather can play a big part in how crowded a park will be.
a quote from this thread http://www.coasterbuzz.com/forum.aspx?mode=thread&TopicID=37311
As far as who might be there at that time - use the meeting calendar. Nice little link to the right of the page :)