well, since you bumped it...
We'll use an example year to make this easier, and then you can figure it out by adjusting for when you plan to enter dental school.
First, when you go to the AADSAS site:
https://aadsas.adea.org/
you will see a link on the left hand side that says create new account. That is the link to open an AADSAS application account for whatever application cycle AADSAS is currently in. Right now, it's in the "entering dental school in Fall 2007" application cycle.
Let's say you want to enter dental school in Fall 2008 though.
In May of 2007, the AADSAS application for students seeking to enter dental school in fall 2008 will come out. The actual date varies by a few days. As soon as the application opens on AADSAS, you can fill it out and E-SUBMIT it. You will also need to send your Letters of recommendation (or committee letter) and your college transcripts to AADSAS. The earlier you do this, the better. It takes a couple weeks for schools to send out transcripts and then a couple days for AADSAS to check that your transcripts match your application.
Once AADSAS verifies your GPA and grades are all correct (this date is known as the verification date) it takes 4-8 weeks for AADSAS to send your application printout to the dental schools you asked it to send it to in your application. During this 4-8 weeks 1000 people on the SDN network will start threads about how slow AADSAS is at sending out applications...people will bitch, joke, whine, cry, and have nervous breakdowns...I suggest going on vacation.
AADSAS will not wait for your letters of recommendations to arrive before sending your letter of rec, but will forward them on as soon as they get them. Again, try to get the LORs in early.
Finally, the Dec 1 and Mar 31 dates you were looking at are school specific dates. For example, if it says NOVA has a submission deadline of December 1, that means you have from may 2007, to Dec. 1 2007 to hit the E_SUBMIT button on AADSAS if you want to apply to NOVA. After Dec. 1st, NOVA will no longer take further applications from AADSAS. So long as you submit your application early, you don't have to worry about these dates...besides, if you are submitting your application in december 2007, or March 2008 and expecting to get into dental school in 2008, well, your chances are starting to get real slim...