Chickens have a pineal gland in their brains, as humans do. This gland detects light, and it's extremely sensitive to amounts of light. It's what regulates our sleep patterns and syncs sleep with wakefulness. This gland in chickens does that, but it also dictates to the endocrine system when to begin laying. Even a blind chicken reacts to light through the pineal gland.
Chickens, having been around for thousands of years, have perfected their reproduction to coincide with spring when tender shoots are coming up that will feed their offspring. That's when most wild animals reproduce, and it takes advantage of this appearance of food in spring to feed their young.
After we reach December 21, the sun will then begin to shift north again. This results in incrementally longer days. The pineal gland senses this and sends a signal to the hen's ovary to begin releasing yolks. It takes around two or three weeks after solstice for the hen's reproductive system to get the message from the pineal gland and to start egg production.
You can expect eggs to appear again by the middle of January, and by February for sure.