If you plan on going at least twice a year, I recommend buying a membership. There isn't any additional cost to you beyond the membership, your adult guest will get in free as well as children, you have access to special exhibitions as and shorter lines, 10-20% dining and the gift shop, and $5 off of parking.
Thursday are free after 5pm for residents. You have to bring ID or a bill to prove residency. It gets busy 16:30-18 on Thursdays for that reason. That info is on the website under FAQ.
The particular event is attended that led to my review was a paid event. An Andy Warhol tribute. There are free days for certain groups, such as Veterans and seniors, etc. but I don't know the dates and times.
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