Blackout Dates are dates the Discount Code cannot be used locally.
Path: Management > Setup > Rates > Discounts > Setup
1. Select the code from the list of discount codes.
2. In the blackout dates section on the right, double click in the empty field.
3. Select the blackout date to apply to the discount code.
4. Use Global Blackout Date to apply a blackout date to ALL the discount codes in bulk. Choose a date by using the calendar button on the right of the field.
5. Click Save.

If a global blackout date is selected by accident, you will have to remove them one date at a time from each discount code in the system by selecting the codes and deleting that one date for each code