They are really great at accommodating. Even with lines, it moves quick. You can just show up.
Day of is fine.
No you don't need to buy them in advance and you get them on site the day of your visit.
They don't seem to limit the number of passes sold, but on a very busy day you may have to wait for extended periods to use the many different VR stations, so you may want to wait and purchase on arrival and after you look around at the wait list length at a few of the more popular games.
You can buy them when you want in.
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