How can I tell when an Opportunity is over?

All Opportunities have a start and end date, and once the end date has been reached the Opportunity will move to past Opportunities. You can still have active Shifts in an Opportunity that is technically over and in the past as well as sign up for new Shifts that are upcoming, but you will not be able to edit your signup.

You can view your past Opportunities under the Opportunities tab of your Homepage.