So, there are no known classical festivals held in Honor of Haides. He was honored in funeral services and during necromantic rites. If you’re wanting to involve some festivals and celebrations in your Haides worship however, here are some suggestions.
Then there’s the Elesusian Mysteries festival which was held in the fall, and lasted three days. It followed the theme of the descent-the search-and the ascent, focusing on Persephone, Demeter, and Hades. Very little is known about the mysteries, because they were secret rites and not ever written on. We do know there’s a strong theme with the underworld and that more than likely the initiated ancient Greeks who took part in the Elesusian Mysteries used items to induce visions. From vase painting we can assume that dance was an important part of the rituals.
We can include elements to honor Haides in Koris Katagogi. Originating in Sicily, Koris Katagogi was a festival specifically to celebrate Persephone’s descent, where other festivals tended to focus on her ascent more than anything else. (Nilsson (1967) Vol I, pp. 463–466).
The New Moon could easily be used for monthly dedications in Haides’ honor. There is some evidence that the Ancient Hellens didn’t celebrate on the new moon, as it was closely associated with the dead. That would make it a perfect time for regular devotions to Haides, and other underworld deities.