Axis of the Earth is tilted by 23.5 degrees due to which there exists different seasons. Moreover Earth revolves around the Sun in elliptical axis.
Winter Solstice happens on 21st Dec when rays of Sun are less on northern hemisphere & hence longer nights & shorter days. Reverse for southern hemisphere. 24 hours of darkness at Arctic circle & 24 hours of sunlight at Antarctic circle.
Summer Solstice happens on 21st June when rays of Sun are are on norther hemisphere & hence longer days & shorter nights. Reverse for southern hemisphere. 24 hours of sunlight at Arctic circle & 24 hours of darkness at Antarctic circle.
Spring equinox happens on 21st March in northern hemisphere & it is autumn equinox in southern hemisphere.
Autumn equinox happens on 21st September in northern hemisphere & it is spring equinox in southern hemisphere.
During both these equinoxes equator receives maximum solar radiations.