I do a lot of cards for silly holidays and crazy "National Days".  These usually fall into the category  Collections » National & International Days » Other.  Where would a shopper find this category using the blue bar across the top of the page?

If you click on All Cards, it and all it's sub-categories are listed, but this is one of the many, many areas where you need to make sure your Card Title, Keywords and Artist's Notes are clear and speak to your card as well as the holiday.
 
People are not too likely to simply go browsing categories for these obscure holidays, they are much more likely to search for them.