continuous open bidding

Depends on if they meet quota or not

the thing with COB is that it isn't really advertised, and they usually only take like a couple of girls through that. Typically, the girls they take already know a bunch of people in the sorority and that's how they get in. It's not something you should really count on unless you have a ton of friends in a particular sorority.

Honestly, if you want a decent chance at getting into a choice sorority, try really hard to make it for fall.
Like someone else said, COB is only for girls the sorority knows personally and really wants. Randoms almost never get in through COB.
Spring is also not ideal because 1) a lot of sororities don't do it and 2) the sororities that do always take like, 5 or so girls. Not only will getting in be much harder due to selectiveness, but you also won't really be a part of your new member class of fall because they all will have already bonded by then. Spring should be a last resort.