Solstice 2025 Calendar. How the solstice and equinox calendar works. The longest day of the year falls on either june 20 or 21 every year in the northern hemisphere.

The equinox and solstice define the transitions between the seasons of the astronomical calendar and are a key part of the earth’s orbit around the sun. When the sun is over the equator you have the equinoxes and when the sun is over the tropic of cancer in the most northern position you have the summer solstice.