What is solstice? Solstice is the moment at which the Sun, in the apparent motion around the Earth, reached the highest positive or negative slope, and then a day in which it occurs. Summer Solstice (the Earth's northern hemisphere) has been traditionally on 21st June, but around 22nd June the Sun reaches declination of +23°27', and then the day is the longest (about 16 hours) and night the shortest.
Winter solstice (Northern Hemisphere) , has been traditional on 21st December but around 22nd December declination of the sun reaches -23°27', when the day is the shortest (about 8 hours). The symbolism of the solstice determines its dual nature: summer solstice begins the summer, but the downward phase of the annual cycle, while with the winter solstice begins winter, but the upward phase, and is associated with winter and Christmas celebrations and all that re-birth, while associated with a summer feast of the Sun (even when he compares Christ with the sun and when it is celebrated as lord of the time).
Astronomical solstice is the moment when the sun reaches its greatest angular distance in relation to the equator. Solstice may be winter or summer. Even though their dates are usually cited as the 21st December and 21st June, the actual moment of solstice in the Gregorian calendar can be a day later or earlier.
Solstice is one of two events during the year when the Sun, seen from Earth, is the farthest on north or south in relation to the equator. The summer solstice occurs around June 21st, Winter Solstice occurs around December 21st. In the northern hemisphere, summer and winter officially begins on the day of summer and winter solstices.