I'll try to explain Millennium Mania as best I can. Last year, CP "invited" you on their website to pay $19 (Season Pass holders must also pay) to attend Millennium Mania. Last year, the dates were Wednesday, Thursday, and Friday of the two weeks after Labor Day (this year it is just the Thursday and Friday of those two weeks). Each of these days, the park was open from 4pm-10pm for anyone that had a special Millennium Mania ticket for that day (you could get them from the website). The only rides that were open were Millennium Force, Raptor, Mantis, Magnum, and Power Tower.
It turned out that there was absolutely nobody at Millennium Mania, the park was empty. Many of the members of this board said that they would be the only ones on the train, and just keep re-riding whichever coaster they were on. There were no lines at all. So basically for 19 bucks you could get a crapload of rides on any of the 5 that were open.
Everyone who went loved it, so they are doing it again this year. While the only thing we know right now are the dates, it will probably be just like last year.
Millennium Mania and Bonus Weekends are probably the best times to get a lot of rides in one day the rest of the year. Halloweekends can sometimes be good also, but you never know, on many occasions they are JAM PACKED (i.e. 10/14/00).
Hope that helps! If you want more on MM, try using the search feature.
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Steuks!!