I'd say 3 months out.
You can go on their website, register, and then check the calendar to see if the day you want is available. It will even tell you whether morning or afternoon swims are available. Just act like you're going to book it, then cancel out at the last step. Then you can log in and go right to the reservation screen to check the day you want.
Booking early will get you a morning swim. Some people like that because they get it out of the way and then relax. It's not necessary, as mine was at 12:20 and everything was perfect.