The celebration begins on the solstice but continues for several days: After all, you have to see with your own eyes that the sun is waning. Us lazy modern folk just trust silly NASA ;) This is an incredibly magical time for the outdoor spirits: they are fully in their element, so to speak and can personally be given their due; We can walk outside now whereas in winter, many outdoor spots are inaccessible by virtue of inclement weather.
This is a time where many are first called to the the night rides. If you've woken up in the morning more tired than when you went to bed, or with aching feet, or sweating, you might have been called (if you think you might be ill, go see a doctor!). Have you remembered your dreams? It might not be time yet to remember where you've been, but you will remember snatches of dream here and there, but not if you try too hard. Remembering can sometimes be like watching something with your peripheral vision- once you turn your head to fully see, you lose sight of it.
Marie Leveau held grand rites on this night as well. Here is an article which describes the ritual held in New Orleans on this night. An excerpt:
Marie Laveau was known for dancing with a snake in her St. John's Eve rituals. The snake was named Zombi, after La Gran Zombi, the main ancestral spirit of New Orleans. Zombi meant "god" or "spirit force" in the Congo, where many New Orleans Voudou traditions have their roots. The loa of Voudou (more commonly called ancestor spirits in New Orleans) are more like intercessors than gods. They are the spirits of powerful people who have died. After the death of powerful or important people, petitioning and "feeding" their spirits through ritual -- and sometimes sacrifice -- along with the power of collective belief, elevates them to become intercessors between humans and God. In New Orleans Voudou, after Papa Legba, the opener of the gates, has been petitioned to open the doors to the spirit world, La Gran Zombi frequently serves as a "channel" to the other ancestral spirits with whom practitioners want to communicate for the purposes of worship, divination and magick.
In ancient Rome, this was the feast of Fors Fortuna, the one who brings luck in all matters. There was a pilgrimage to Her temple which included walking by the Tiber River (funny how the rivers figure into it), much revelry and drinking. There were oracles dedicated to Fortuna which were especially consulted on this night, and this feast is still popular for its divination traditions today!