When organizing an innovation day/Hackathon, it's worth planning ahead. Communicate often and early that we intend to run an innovation day and give the specific dates and timings, repeatedly.

This should probably start at least several months before the intended date. Put it on everyone's calendar far enough ahead of time so that everyone is guaranteed to be available. An example format with the suggested timings is as follows:

1. Hold a 2-hour liftoff session, approximately 2 weeks in advance of the innovation day. Treat this like a mini-team liftoff session, with a mission briefing and working agreements (the rules that we'll operate under). People will need to find teammates and start thinking of ideas to work on; this event should trigger those thought processes.
2. If you're awarding prizes, make sure you've organized them.
3. Hold another session just before the team starts work on their innovation project. There will be a bunch of small teams and maybe even some people who have chosen to work as individuals. Give each group/individual a small timebox to present the idea that they will be working on. From this point onward, everyone commits to demoing the results in the event showcase at the event finale.
4. Make sure you have a clear start and end of the event. For example, from 10 a.m. Monday until 10 a.m. Wednesday. Some people like to kick off on the first day with a morning breakfast, others choose to wait until 12 p.m. The midday timing is especially good for 24-hour events, as it allows people to get some shuteye in their normal sleep cycle. 
5. Circulate among the teams during the event to check in with them. Make sure enough drinks and snacks are available. Make sure they have the things they need.
6. At the showcase, stick to the timebox for each demonstration and allow time for a Q&A.
7. If you're awarding prizes, have a small break after the showcase to collate the judges' scoring and feedback. Reconvene everyone and hold a separate prize giving.