When the Sun reaches the highest or lowest regions in the sky, Solstices happens.
Explanation
A solstice is characterized by the poles of the planet whose inclination is away from or towards the star where the planet orbits.
When taking our planet into consideration, the poles are, in the North, the Tropic of Cancer and in the south, Tropic of Capricorn.
Solstices occurs twice in a year.
One in June and the other in December which will be the Northern hemisphere's longest day and southern hemisphere's shortest day.