The sun is lowest in the sky during the winter solstice. This occurs around December 21st or 22nd in the Northern Hemisphere and June 20th or 21st in the Southern Hemisphere.

Here's why:

* Earth's Tilt: The Earth is tilted on its axis. This tilt causes different parts of the Earth to receive varying amounts of sunlight throughout the year.

* Winter Solstice: During the winter solstice, the hemisphere tilted furthest away from the sun experiences its shortest day and longest night. This means the sun's arc across the sky is lower and shorter than at any other time of the year.

So, while the exact date varies slightly each year, the winter solstice marks the time when the sun is at its lowest point in the sky.