When does The Grim Adventures of Billy & Mandy return?

The Grim Adventures of Billy & Mandy is an American animated television series. The series follows the titular two children named Billy and Mandy who, after winning a limbo game to save Billy's pet hamster, gain the mighty Grim Reaper as their "best friend forever" Grim, who is reluctant to serve the two children, has access to supernatural items, spells, and other abilities that often lead them to interact with otherworldly environments, characters, or situations.
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.