Hi Jeni, Go to the latest challenge scroll down the page you will see a section named "Upcoming Challenges" with a date. On this page you will see this:
"UPCOMING CHALLENGE
Challenge #468 (07.05): Drag-and-drop interactions. This will be a general drag-drop challenge, so you can share anything you like."
I believe there are no "close" dates, you can do any challenge at any time. They are added to the Recap overtime, not sure if that has a cutoff date. If you want feedback and the max. amount of people to see it posting within the first couple of days of the start date seems to work best.
P.s. Something I often forget to do that.
If you want to be updated on what is happening in a particular challenge you can subscribe to the challenge by clicking the Subscribe button at the top of the challenge page - you will receive emails of the postings and comments for that challenge.