Note: Now that Workflows are live in all accounts, you can do everything that Triggers and Campaigns do (and more!), all in one builder! Click to learn more about Workflows.
How & When To Use The Event Start Date In A Campaign
The Event Start Date field allows you to build automations around a specific date & time. Once you set the Event Start Date, all the items in that campaign will trigger in relation to that day/time.
So for example, if you were running a webinar on January 20th, you would be able to build a campaign with the Event Start Date set to January 20th, 2pm and then add items in the campaigns like emails and text reminders that go out the day before and the hour before by setting the timing of those items to "Before 1 day", "Before 1 hour", etc.