I've no idea how they will celebrate National Watermelon Day in the US. Just eat a lot of it, I suppose, and perhaps some poor soul will have to dress up in a puffy green outfit. 

If you'd just like to get into the eating part, here are a few ideas to help you celebrate: 

Like most fruits (except banana*) watermelon works really well in salads. It's especially good with salty cheese like haloumi or feta. I often just chuck a load of diced watermelon into a bowl with olives, crumbled feta, basil and some balsamic glaze for a quick and easy supper.

If you've a bit more time, Josh Katz's salad makes a wonderful starter or light main.  

Israelis eat a LOT of watermelon - it's so delicious, how could you not? Chef Barak Aharoni gives it an Asian vibe, as the base for cured tuna in this delicious starter.

This fabulous meaty and melon main from Lisa Roukin, pairs rich duck with sweet, refreshing watermelon. A perfect Shabbat dinner main course for this balmy weather.   

* For the record, I'm not a banana hater. It just doesn't work for that whole sweet/savoury thing. Mush it on my toast with peanut butter; heat it up over a fire and serve it with ice cream and salted caramel sauce or freeze it an blitz it into a 'healthy' ice cream. Just leave it out of my salad.