Short answer: In the Northern Hemisphere, days begin to get longer after the winter solstice, which occurs around December 21st each year. The increase in daylight is gradual, with the rate of lengthening typically noticeable in late December through January, and continuing until the summer solstice in June. Details

• Why it happens: The tilt of the Earth's axis relative to its orbit around the Sun means that after the solstice, the Sun climbs higher in the sky each day, lengthening daylight hours.
• Key markers:
  • Winter solstice: the shortest day of the year (around December 21, when the Sun reaches its southernmost point in the sky).
  • Daylight begins to increase soon after that solstice and continues to lengthen toward the vernal (spring) equinox in March, and then to the longest day at the summer solstice in June.
• Practical observations:
  • In temperate latitudes, you will notice the sunsets gradually happening later, and mornings may start to brighten a bit earlier as days lengthen.
  • In far northern areas, the change is still present but can be less perceptible on a day-to-day basis due to already long nights around the solstice.

If you’d like, specify your latitude or city and I can give an approximate daily daylight length trend for the coming weeks.