Demo Day is the public reveal of an accelerator cohort. Each company gets a short pitch slot, typically 2 to 4 minutes, in front of an audience of investors, press, and other founders. The event is both a fundraising sprint and a credentialing moment.
The Y Combinator Demo Day model has been copied by hundreds of accelerators. Top YC companies often complete their post-Demo-Day round within days; for many cohorts, the round is largely set in the days immediately after Demo Day.