Halloween is a pagan holiday celebrated during the Fall, usually in October. Across the globe, traditions will vary, but one common theme is the act of celebrators dressing in costumes and children going trick-or-treating. Non-celebrators may hold religious meetings or simply choose not to partake in trick-or-treating.
For the full article on Halloween, visit the Wikipedia page here.
In Season 1 of the original Danny Phantom series, the episode, "Fright Night," takes place on Halloween. In it, Dash and Danny clash heads and Mr. Lancer bears witness, and so he assigns them each one room in an abandoned house that is designated as the official Haunted House each year. Determined to beat Ms. Tetslaff's display last year, Mr. Lancer enlists the boys as his decorators and whomever makes the scarier room will be exempt from detention. Meanwhile, Dash and Danny make a side-bet: if Danny wins, Dash will streak across the school parking lot in his underwear, but if Dash wins, Danny has to eat Dash's old, discarded underwear in the school cafeteria.