On the Promote page of your event, click on the "Select the promotion start date" button.
You will then see a calendar dialog where you can select your start date.
- You find the button after clicking on “Promote this event” on your event page:
- The end date of the promotion will be equal to or less than the date on which the event ends.
Maybe you want to take a look also to our article: What can I expect for a paid promotion