The Halloween series consists of songs uploaded on or close to Halloween each year from 2008 to 2012.
Each song (except for Halloween Monster Party Night) has the numbers "1031" somewhere in the song. This is read as "Halloween". Most of the songs also have "lulilu lilala" sung, sometimes more than once. It is usually mentioned as a "spell" or "cursed words".
Much like the last song, it is sung by a group of monsters having a party surrounded by the dead and other monsters. At the end "you" utter the spell (lulilu lilala) and are turned into a monster yourself.
This song started the 1031 and luliu lilala pattern.