The phrase “shortest day” usually means the day with the least daylight, which is the winter solstice.
General answer
In the Northern Hemisphere, the shortest day of the year is typically on December 21 or 22, known as the winter solstice. In the Southern Hemisphere, the shortest day is usually on June 20 or 21, when that hemisphere is tilted farthest away from the Sun.
Specific to 2025
In 2025, the winter solstice (and thus the shortest day by daylight) in the Northern Hemisphere occurs on Sunday, December 21. The solstice moment is at 15:03 UTC (10:03 a.m. EST), but most people refer to the entire date, December 21, as the shortest day.
