Compos at a demoparty are usually held during the afternoon and lasting after midnight, usually on the last whole day of the party.
Each compo has its designated deadline, to which the competitors must deliver their entries, either via traditional physical media or through the party LAN. The purpose of the deadline is to allow the compo crew to prepare the assets of the competition: the crew makes sure that each entry runs correctly, perhaps records them to video in case of alternative platforms.
The crew also performs preselection on the entries to be able to fit in a given timeframe with the competition, and also to maintain a quality level for a given competition. (Preselection at larger parties has always been a controversial standpoint, given the versatile stylistic state of the demoscene productions.) While deadlines are usually explicitly stated at demoparties, most parties are rather flexible considering release handins, especially if they are aware of a release's high quality - demogroups who can't finish the production before the deadline often create "placeholder" versions of demos, which serve the purpose of reserving the place for the production in the compo