You will need to add items from The Knot Registry Store before your event to have them as part of your eligible purchase, as is stated in the Post-Wedding Discount terms and conditions. So you can add items up to and including on your event date, however, items added after this date will not be eligible for the discount.
As a reminder, at least one of these gifts from The Knot Registry Store must also be purchased before your event for the Post-Wedding Discount to be activated. The best way to ensure this happens is to share your registry with your friends and families before your big event!