For the Northern Hemisphere:

The first solstice in any given year is the day when the Sun provides the most amount of daylight hours. This happens on or about the 21st of June.

The second solstice in any given year is the day when the Sun provides the least amount of daylight hours. This happens on or about the 21st of December.

The opposite is true in the Southern Hemisphere.

The solstices and the equinoxes, are celebrated in Neo-Paganism, Wicca and some Trads in Traditional British Witchcraft, as well as in some cultures and other Pagan religions.