keep in mind random slots open up --> there is an email notification service that will email you if a slot opens in a timeblock you specify (you can select up to 5 windows of 30 days each).
This of course leads to a deluge of emails if they all of a sudden open up a buncha slots for an entire month but can be helpful if you needed to switch to something earlier.
be careful though and check with your programs to see when their deadlines are. Many will require you to record a passing score before starting