The Shortest Day of 2018 - 21st Dec. - Winter Solstice
What is a Solstice ?
What is an Equinox ?
Reason ?
Due to the tilt of Earth's axis around its orbit
What is a Solstice ?
A Solstice is a point of Earth's orbit around Sun, at which Sun is at the greatest distance from the Equator
In Summer - Days are longer - June 21
In Winter - Nights are longer - Dec. 21

What is an Equinox ?
An Equinox is a point of Earth's orbit around Sun, at which Sun is at the closest distance from the Equator. Equinox has equal days and night, which also occurs twice in a year; March 20 and Sept. 22

Reason ?
Due to the tilt of Earth's axis around its orbit