If you want to buy tickets on their website, you have to choose a date before being able to check-out, currently, they only let you choose a date up to March 31. I wonder if they also limit the number of visitors per day and if there is a problem if you just show up there, no tickets in hand. Interestingly, Orlando Fun Tickets sells undated tickets, saying they are valid for any date in 2021 at a $6 discount over the prices at KSC. But then, Orlando Fun Ticket also says the ticket allows you to access the guided bus tour (which isn't running at this time), so go figure. :-/