The summer solstice is an annual event occurring in midsummer, around 21st of June in the northern hemisphere and 22nd December in the southern hemisphere. It is the longest day of the year and is celebrated as a holiday by Night People.
According to Ash Redfern, the summer solstice is regarded as being "very magical" by Night People and has been celebrated by them for a long time - it is not specified how long exactly. Ash compares the summer solstice to Groundhog Day, citing the solstice as being the Night World equivalent. Night People often celebrate the solstice by throwing parties, known as solstice parties, where Night People of all kinds gather together. It is said that the celebrations reach a peak at around midnight. The Night World Elder, Thierry Descouedres, throws a massive party at his mansion in Las Vegas every year on the summer solstice; it can be assumed that similar parties or gatherings are held across the nation.
The summer solstice occurs a few days after Poppy North is changed into a made vampire by James Rasmussen. After being convinced by Ash to leave James for his own protection (Poppy being an illegally made vampire), she travels with him to Las Vegas. Ash takes her to Thierry's mansion where they observe the celebrations from an anteroom. Blaise Harman also plans to attend the party. The guests include a shapeshifter named Sekhmet, a "witch of renown" named Circe and a made vampire with the appearance of a four year old (who could possibly be Timmy). Ash then attempts to drag Poppy into the main party room, intending to expose her as a renegade vampire. Fortunately, Poppy is able to resist, fighting back using a psychic attack. James and Poppy's brother Phillip North soon arrive at the mansion, having deduced that Ash would bring Poppy there after learning that Thierry was throwing the Solstice party, and Ash lets them walk free, claiming he'd only been joking about turning her in. James, Poppy and Phil are able to slip out without anyone at the party noticing their presence; on the way back to El Camino, it is deduced that Poppy and Phil are actually lost witches due to Poppy's skill at telepathy and prophetic dreams, meaning she has every right to know about the Night World and is not illegal after all.