The summer solstice, it is the longest day of the year, date of the calendar where the night is the shortest. In the northern hemisphere, the summer solstice falls generally on June 21st (and sometimes on June 20th). But it is not always the case. Indeed, our civil calendar (the Gregorian calendar) account 365 or 366 days and is not perfectly synchronized with the cycle of rotation of the Earth around the sun (365,2422 days to be precise!). From where of small gaps, certain years …
? In June 21st: it is, in the great majority of the cases, the official date of the summer solstice.
? In June 20th: it had not arrived since 1896, it had never arrived at the XXth century, but the summer solstice had for official date June 20th in 2008, in 2012 and in 2016. It will be of new the case in 2020 and, in everything, 47 times on 100 in the XXIth century.
? In June 22nd: the summer solstice arose at this date for the last time in 1975. And it will not reproduce before the XXIIIth century! Meeting in 2203, on 2207, on 2211, on 2215 ...
? In June 19th: Watch out, we reach here the highlight of the cosmic rarity. The summer solstice in the northern hemisphere will fall EXCEPTIONALLY on June 19th in 2488! It will be the very first time since the creation of the Gregorian calendar in 1582.
To know all the official dates of the summer solstice for the last centuries and for those to come, thus take a walk here: https://www.imcce.fr/newsletter/docs/Solstice_e te_1583_2999.pdf
Please take time, to digest all this and, when you will organize a collective journey to celebrate the summer solstice, ask for help a specialist of the accommodations of group.