Hi and welcome to the Community @newspin! 

When you create a marketing campaign you'll be able to set the number of days/weeks/months the coupon will be active. It's not currently possible to se a specific date but I definitely see how that would be a useful tool!

To set the amount of time your coupon will be active: 

1. From the Design section of your campaign click Edit Coupon.
2. Scroll down to the section 'Expires In'.
3. Click on the dropdown menus to set the number of days, weeks or months your coupon will expire after. For example, if your art fair is running over a long weekend, select 3 and select days: The coupon will expire in 3 days.

You can also schedule your campaign to run for a future date. That way you don't have to send the campaign while your getting ready for your event! 

If you have any other questions please let me know.