When does The Grim Adventures of Billy & Mandy return?

The Grim Adventures of Billy & Mandy is an American animated television series. It follows Billy, a dimwitted, happy-go-lucky boy, and Mandy, a cynical, remorseless girl, who gains Grim Grim, the mighty Grim Reaper, as their "best friend forever" Grim has access to supernatural items, spells, and other abilities that lead Billy to interact with otherworldly environments, characters, or situations. During its run, the series won two Emmy Awards and one Annie Award.
The series is centered around the exploits of Billy (Richard Steven Horvitz), an idiotic and happy-go-lucky boy; Mandy (Grey DeLisle), a cynical, cold-hearted and grumpy girl; and Grim (Greg Eagles), a Jamaican-accented Reaper. Billy and Mandy use Grim's supernatural abilities to venture into supernatural locations or environments, such as the Underworld, or the Netherworld, inhabited by an assortment of grotesque monstrous beasts. Famed fictional monsters including Dracula, the Wolfman, and the bogeyman are also comically depicted in the series.