Joyous_Night is one of the members and nights of The Night Coalition.
Other than being affiliated with The Night Coalition not much is known about him, but just like Horrid_Night and Dauntless_Night, he thinks of Albert as a sacrifice. It is presumably that he hates the days as well.
When Albert and Great_Day are talking in front of a portal, Great_Day states he feels the presence of darkness. Then Joyous_Night and Dauntless_Night both appear saying "Welcome home sacrifice mrflimflam and "this is torture".
Great_Day then tells Albert to run into the portal, which he does. It is unknown what happens afterward.