This is your only chance to:
(a) apply for Bagley if you are out of the area
(b) apply for Montessori
It's also an opportunity to sign up if you are in the area. Although this can be done at any time, since you're guaranteed a spot in the contemporary program if you live in the Bagley attendance area, and it is a great idea to give the school an idea of who is coming. It lets SPS begin the process of assigning spots to the people who apply during open enrollment.
Scan through old posts for more information about the enrollment process. Look for the label 'registration' and take a spin through this post from a prior year.