I had a lot of fun with this writing prompt. Make up your own holiday, rich with customs and traditions. Then tell the story of how the holiday came to be, or the story of one person or family’s celebration of it.
If you’re stumped for ideas, you might find some inspiration here: http://www.kidzworld.com/article/4456-weird-and-wacky-holidays. I’m a little disappointed I missed Take Your Houseplant for a Walk Day!
Also, it’s apparently not too hard to get your holiday formalized by the United Nations. Bonus points if your day can get us off of work.