Chillagy refers to " Trilogy" which is a set of three works of art that are connected, In this case you chill for 3 days straight. There are no breaks and you must remain with atleast 2 other people.
Guy 1: This weekends gunna suck!
Guy 2: Yo lets have a chillagy.
Guy 1: Im down for a chillagy.
Guy 3: yeeeeeeeeeeeeeeee