I went at 9am (as soon as it opened). The wait to get through the multiple lines (tickets, security, etc) and the elevator rides took about 15 minutes).
Even on busy days the sky deck never seemed crowded, despite the lines of people waiting to get in. You may have to wait in line some time but it's worth it. You may want to check the weather forecast first so that you will have clear views. I have seen the top of the tower in low hanging clouds before and then you wont be able to see anything.
Week days early in the morning. It opens at 9.
Week days in the morning, especially now with school back in session and everyone else at work.
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