A closing date is important, because after this you can send out the personalised schedule to your attendees (the final schedule email).
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Go to Dashboard > click Edit event > scroll down in the page. Update the Event closes date > scroll down to the bottom of the page and click Save. 

The closing date is usually two days before your event at 23:59h. 

An example:

26th of September - 23:59 seating closes
27th of September - in the morning send out the final schedule
28th of September - EVENT