I recently hosted a successful summit called the grow and profit online summit a few months ago. Reasons it was successful include:
Good timing.
Strategic promotional activities.
Effectively working with experts.
There’s no specific date or time to stop registering people to your event. You can keep on getting last minute subscribers right up to the last five minutes before your launch. Some techniques you can use include:
Social media pages
Carousels
Message your mailing list