The main celebration is in the Summer Court although all the Courts celebrate it. The Solstice indicates when day and night are equal, it's a time of neutrality, when everyone can take down their hair and simply enjoy being faerie.
The Summer Solstice is also celebrated in the Mortal Lands with small gatherings in the center of the villages, nothing religious, though the Children of the Blessed would try to convert people, just some shared food, donated ale and maybe some line of dancers. The only thing the humans celebrate is a day's break from the long summer days of planting and tilling.
The Spring Court
Although the Summer Solstice is celebrated in all Prythian, we only know how it's held in the Spring Court where the manor's gardens are decorated with ribbons and streamers, and in the hills bonfires are lit and many poles are raised. A party is held during which tables and tables of food are prepared and the air is filled with light and joyous and mirthful music played by merry fiddles. Faerie wine is served and lines and circles of dances begin. As Lucien Vanserra says in A Court of Thorns and Roses the Solstice it's a time for dancing and excessive drinking and dallying.
In the Spring Court during the Summer Solstice is also hold a religious ceremony that takes place two hours before dawn. During the ceremony an High Priestess stand before a rudimentary stone altar bedecked in flowers and the first fruits and grains of summer, then begin an endless string of prayers and rituals during which sacred wine is poured, with the blessing of the harvest goods on the altar, and beseeching the sun to rise. At a certain point the High Lord of the Spring Court is summoned to the altar to light a candle for the souls extinguished in the last year, and to bring them back into the light's embrace when the sun rose. Sometimes someone in the crowd is called to recite the final prayer for the warriors who fought for the people's safety each day. The ceremony culminates with the High Priestess standing with her arms upraised, the sun rising and filling them with light.
While Feyre is living in the Spring Court of Prythian she sees the entire court is preparing to celebrate the Midsummer Solstice and is invited to attend. According to Tamlin, before the arrival of Amarantha, the Summer Solstice was a great party held in the Summer Court but for forty-nine years it has not been celebrated there anymore so that the inhabitants of the Spring Court are having their own party on the grounds of the manor.
For this occasion she was dressed in a flowing, cornflower-blue chiffon gown, her hair was left unbound but she wore a garland of pink, white and blue flowers above her head.
During the party Feyre gets drunk on faerie wine, against Lucien's advice not to do so, as it causes her great euphoria and disinhibition so she dances all night with the faeries around a large bonfire while Tamlin plays his fiddle. Later he takes her to a small clearing to see the will-o'-the-wisps as they dance together between them and share a few kisses for the first time. Then they watch the sunrise.
Feyre attends the Summer Solstice religious ceremony held by Ianthe. Tamlin, Lucien, Jurian and the Hybern twins, Brannagh and Dagdan are also present. At the end of the ceremony when the sun should have hit Ianthe's raised arms Feyre makes sure that the sun hits her in the High Priestess' place, releasing some of her Day Court magic and starting to glow as brightly as the sun. At this Lucien takes Feyre's hand and presses her fingers to his brow, kneeling before her. After this Feyre earns the title of Cauldron-blessed.