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 hold 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 hold during which table and tables of food are prepared and the air is filled light and joyous and mirthful music played by merry fiddles. Fairie wine is served and lines and circles of dances begin. As Lucien 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 take 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 the 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 safety each day. The ceremony culminate with the High Priestess standing with her arms uprised, the sun rising and filling them with light.
A Court of Thorns and Roses
During A Court of Thorns and Roses Feyre participates the Summer Solstice festivities. 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, and dances for Tamlin while he plays the fiddle. Later he takes her to see the Will-o'-the-wisps, spirits of air and light. They dance together between them and share a few kisses for the first time. Than they watch the sunrise.
A Court of Wings and Ruin
During A Court of Wings and Ruin Feyre attends the Summer Solstice religious ceremony hold by Ianthe. Tamlin, Lucien, Jurian and the Hybern twins are also present. At the end of the ceremony when the sun should have hit Ianthe's raised arms Feyre make sure that the sun hit 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 press her fingers to his brow, kneeling before her. After this Feyre earns the title of Cauldon-blessed.