Kay Nielsen concept art
Celebrated in Germanic nations (Germany, Finland, Sweden, Estonia, and Latvia) by pagans who burn fires in honor of their deities and by Christians who burn fires to keep the demons away. Either way: fire! This is the night that the barrier between the world we live in and the realm of the supernatural is said to be particularly thin. According to Fantasia, of course, this is the night that the demon Chernobog (or Chernabog) rises from the peak of Bald Mountain and calls forth a host of ghosts, demons, and monsters to dance for his evil amusement.
Kay Nielsen concept art.
Chernobog is a death god from Slavic mythology, his name meaning "black death." In the intro to "Night on Bald Mountain," Deems Taylor refers to him as Satan, a much more familiar devil and one whose name comes with a lot more baggage. Somewhere along the line, Disney decided to start referring to the character in its merchandising as "Chernabog." Early animation notes and model sheets (from designer Kay Nielsen and animator Vladamir "Bill" Tytla) refer to the character alternately as "Chernobog" or as "the Devil."
Kay Nielsen model sheet.
Have a happy Walpurgisnacht, dear readers. Careful dancing naked around the fire. If you're concerned at all about bad weather or singing your naughty bits, you could also consider celebrating by staying in and watching Night On Bald Mountain instead.
More on Chernobog here.