Can someone explain to me why today is Hekate day?? I feel like I’m missing something?

It’s a modern thing. 

On the Latin calendar, there’s the Festival of Diana Nemorensis on August 13th, and in a lot of Hellenistic sources, Diana and Hekate get conflated. Soooo… people have turned August 13th into a festival for Hekate. 

To learn more about the original festivities, read Alfoldi’s Diana Nemorensis.