|Sunrise on the Winter Solstice at Stonehenge|
To early peoples, who (in the Northern Hemisphere) noticed the days getting shorter as the sun moved south on the horizon, it was good to know that the trend would reverse and the days would get longer again. They did not know that the reason was the tilt of the Earth's axis and the fact that it was pointed away from the Sun. All they knew was that the nights got longer.
Memory told them that the same thing happened last year, and the Sun always paused for a day, and then returned. To the naked eye, it was not always easy to be certain that the Sun was returning and the days were lengthening; four days was a sufficient span to be certain. Therefore, although the 21st of the month (by the reckoning of people who used the ancestor of our current calendar) was the Solstice, it was the 25th of the month that was celebrated as a certainty of the return of the Sun.
Some cultures designed ways to be certain that they had reached the "darkest part" of the year. The arrangement of rocks on Salisbury Plain that we call Stonehenge was apparently designed (among other reasons) to mark the Solstice.
The day that they were certain the Sun was "returning" was a time for feasting. With winter established, livestock were slaughtered because they could not be easily fed during the next few months; fresh meat was now plentiful and it was a time for a mid-winter feast before hunkering down to wait out the harsh cold months until planting should begin.