Final answer:
The religious festival celebrating the shortest days and longest nights in the United States would be held in late December, around the time of the winter solstice, which is typically around December 21.
Step-by-step explanation:
The religious festival that coincides with the time of year when the days are shortest and the nights are longest in the United States would be held in late December. This period aligns with the winter solstice, which typically occurs around December 21. During this time, the tilt of the Earth's axis is such that the Northern Hemisphere is pointed away from the Sun, resulting in shorter days and longer nights.
Many different cultures have celebrations around the time of the winter solstice. For example, Stonehenge in England is believed to have been used as an ancient device to anticipate the shortest day of the year. In contemporary times, we continue with various holiday celebrations around the winter solstice.