<human>:  What is the date of Christmas Day in 2021?
<bot>: Christmas Day in 2021 is on December 25th. The date of Christmas Day is determined by the Gregorian calendar, which is the most widely used calendar in the world. The Gregorian calendar is a solar calendar, which means that it is based on the cycle of the Earth's orbit around the sun. The Gregorian calendar is used in most countries around the world, including the United States, Canada, and the United Kingdom.