I would also imagine that the Day of Little Demons would have especially strong followings in certain districts and, even more specifically, on certain streets. Some parts of the city would see people going about their daily lives and maybe telling their boss to sod off. Others would see people having orgies in the street and a generally crazy, exultant time. It would vary.

Holiday Ideas:

Day of Life:
While originally started as a holiday by the Holy Hands, it currently resembles what it once was very little. Originally, it was a day when healers who were not of the Holy Hands would join up for a day and walk into the poorest parts of the city and stand on street corners healing sickness, curing blindness, and healing injuries. Many such healers, however, took to standing in the Necropolis, offering "healing" to every undead resident that passed. The undead took offense to it but, rather than complaining about it, they decided to go in a whole new direction with it.

The undead started celebrating the lives they once had and what was left of them. It became a day of gluttony in the necropolis: those that were still physically capable of it would eat food they couldn't digest just for the hell of it, vampires would drink whole kegs of blood while ghouls would feast on body after body, saved up for the celebration. Those still physically capable of it would host orgies and have other pleasures for the senses. The entire Necropolis would have bright paper lanterns and colourful decorations strung up, and undead would wear colourful costumes as they marched about, dead things celebrating life.

To this day, the Holy Hands continue with their holiday as per their original intentions, even standing and offering final death to smiling necropolitans dancing past in bright dresses and suits. Most people, however, know it as a festival of ecstatic undead and bright colours, where food and drink perfectly suitable for living consumption is plentiful and free in the necropolis. Poor people and people who love to party flock to the necropolis on this day and slowly filter back out over the next few weeks as the food and drink suited for human consumption is eaten or goes bad.

Solar Eclipse:
Though I don't think that they would necessarily have any sort of official names, I think that various eclipses would be very short, small-time holidays in the city. On these holidays, creatures with a sensitivity to bright light or a weakness to sunlight would flock up to the surface of the city, forming small, celebrant flash-mobs which would dance and sing in the heat of the day and the dark of the night. They would only last as long as the eclipse, or shorter for those that can only be out during the full eclipse. At the end of that time, they would flood back into the under-levels of the city as if they had never been there at all.