The summer solstice, falling on or near the twenty first of June, gives the month its religious shape. Modern Pagans keep the longest day as Litha or Midsummer, one of the eight stations of the Wheel of the Year, with vigils from sunset to sunrise and gatherings at monuments aligned to the solstice sun, of which the dawn crowd at Stonehenge is the most famous. Across northern Europe the older midsummer fires survived christianisation by attaching themselves to the eve of John the Baptist's nativity: St John's Eve on the twenty third of June is still a bonfire night from Scandinavia to Spain, and Scandinavian Midsummer remains one of the year's chief festivals under a nominally secular name.
In the southern hemisphere the same date is the winter solstice, and it carries the year's great sun festival of the Andes. Inti Raymi, the festival of the sun, was the Inca celebration of the solstice and new year, suppressed by the Spanish in 1572 and revived in 1944 as a vast annual re enactment at the fortress of Sacsayhuaman above Cusco, where the ceremony petitions Inti, the sun, to return from his farthest point.
The month also carries observances of a newer kind. World Humanist Day, kept on the solstice itself, was established by the humanist movement as a deliberate reclaiming of the sun's turning for a non theistic celebration, and the movable feasts of Corpus Christi and the Sacred Heart, counted from Easter, fall in June in most years.
Sources The Triumph of the Moon: A History of Modern Pagan WitchcraftRonald Hutton with Encyclopaedia Britannica (online academic reference)Encyclopaedia Britannica editors, Inca Religion and Customs (Historia del Nuevo Mundo)Bernabe Cobo; translated by Roland Hamilton