There is a number of sessions allocated for each exercise or part exercise. The deadline for the work is the end of the last session for that exercise (or part). However, the students are entitled to have an extension, normally to the start of their next session for that same course unit, but they must ask for it towards the end of the session. Although they are supposed to ask, it's a good idea to solicit extension requests in the last 15 minutes of the session for those who need it. (Giving an extension is the simple process of putting a tick in a column.)
It is important that the students ask for the extension, rather than you giving it automatically, so that they don't just turn up for an attendance tick, and then disappear (if they did, it is likely that they were having difficulties and were not yet facing up to these). It also reinforces the perception that they should, in theory, have completed by the original deadline, and they are being given extra time to complete it in their own time. It has been shown from previous years that this really works: many students do lots of work before the first session scheduled for an exercise! (At least to begin with.)
However, the above is the `norm' and there is variation in some course units. Some deadlines do not have extensions, some have extensions to the start of a session later than the next one, and some have automatic extensions as long as the student attends the deadline session.