I heard that they were first mentioned in Meng Yuanlao's Eastern Capitol: Dreams of Splendor but do we know anything about their history before that? And what exactly does "Open Door" mean in this context?
Generally believed to originate in the Song Dynasty where the book you referenced came from. Although the Song Dynasty text mentions eight including Alcohol but you know isn't that necessarily so it was removed during the short lived Yuan Dynasty.
Opening the door just means start of a day whether it is entering your house or leaving your bedroom you need to open the door.