The web site will tell you when you can start getting tickets. However tickets are usually three months out. Ex. tickets that are available in February are for May so you have to plan early. They do however have a few first come first serve tickets starting at 6:00 a.m on certain days.
If those methods don't work, you can show up weekdays at 1 pm to see if they have same day openings.
Same day tickets are available beginning at 6:30AM. Tickets are given out the first Wednesday of each month. I believe the next batch of tickets available next month is for April or May.
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