Birth of the son = rebirth of the sun as we pass the solstice
Stringing up lights is symbolic of the life giving light which begins its slow inevitable return on the solstice. If we just hang on long enough, the spring will come.
It’s a celebration of the sun and a celebration of light and life.
So was it just a coincidence that jesus was born on this day which happens to have been celebrated for thousands of years already and just happens to coincide with these changes in the seasons?