Sorry, @creative-click-photography, for not responding sooner. Here’s the answer to your question:
For each opportunity, a deadline is set so folks will be sure to submit. Sometimes, that deadline is extended due to a lack of participation or because the project timeline changes behind the scenes. However, an extension is always the exception. The more I get accustomed to how the assignments work, the less and less assignments will be extended.
So, having said that, there is a little bit of a risky “grace period” that I don’t necessarily broadcast: true, the written deadline is the deadline, BUT, if the actual thread isn’t closed (like, grayed out in the list on the main STS page) then technically you can still submit photos while that thread is open even though it’s technically after the written deadline. This grace period could be hours, days (like, over a weekend) or even weeks (like this time, over the holidays).
So, just as a little example: if the written deadline was the 5th, and I don’t actually close the thread until the 7th, than submissions made on the 6th are perfectly fine. It’s just that you never know (and I really never know either) just how long that grace period will be.