Bottomless Gravy Day is one of the events that takes place at Peach Creek Jr. High. It was first mentioned in the episode "Cleanliness Is Next to Edness," when Ed thought that it was a school day and that it would be Bottomless Gravy Day in the cafeteria. Because of this, he woke Eddy and Edd up on a Saturday. When Eddy found out it was the weekend, he became annoyed and threatened Ed. Ed then used this event as an excuse; specifically, he said that his belly told him it was Bottomless Gravy Day.
It can be assumed from the title that the cafeteria serves as much gravy as anyone requests. It seems as though Ed is the only one interested in this event. In "All Eds are Off," he mistook a barrel of butterscotch pudding (there for Fun With Butterscotch Pudding Tuesday) for gravy, and assumed it was this event. Because of this and his gravy withdrawal at the time, he dove into the barrel and nearly lost the bet. Strangely, this was how it was discovered that Ed is allergic to butterscotch pudding.