It's important to check that the various tools you're using for your event are ready to go day-of. A dry run of the whole event largely can also help your team be prepared to handle everything and let you know about any bottleneck that might create problems later on.
Here is a check-list of items to ensure day-of:
Check-in is ready and up (You have all the information you might need later on)
Streaming platform is ready to go
Opening ceremony ready to go
Sponsors are on your chat platform with their assigned roles
The schedule is accurate and visible to attendees
Hackers have access to relevant channels on the chat platform
Mentors and judges are confirmed
Internal tasks are assigned to individual members of the organizing team