I think the short answer is "any time before the deadline."
You could probably over analyze when the perfect time to submit would be. Is it the day the application is released, so you make a great impression when the reviewers are fresh. Or is it midnight on the last day, because the reviewers will have gone through a lot of garbage by then and yours will shine?
I don't think it matters. Who's to say if they review them in the order received? Many programs may not even start the process until after the deadline closes.
My general understanding is that, in most programs, they strip down the applicant pool to a certain number (100? 75?) and then the faculty all weigh in and rank them. So, you don't have to worry that your spot is taken by an early submitter.
My instinct is wait until the end. If your material is really ready to go in at beginning of the application window then there's no harm in reviewing it again...and again. It would be horrible to submit early only to find careless errors, typos etc. The decision will be made on the quality of your portfolio (though I know firsthand focusing on things other than the portfolio provides a nice distraction). Goodluck!