None of the past boxes are available to me aside from the ones containing coupons, and I’m a designer. Once the day has passed and the downloads aren’t available it won’t be clickable anymore.
I know it’s frustrating to miss a day, but it’s a huge and varied amount of packs. Just try to enjoy what you got! And if you miss part of it, just know that the packs are spread out through the month so you will have a better chance of receiving one of each type. Also, feel free to share the link to the advent calendar on FB or Instagram with your friends so they can enjoy it too! Set a reminder on your phone each day to check it so you don’t forget.