The calendar we use today is called the Gregorian calendar. It was introduced by Pope Gregory XIII in 1582. The calendar is based on the solar year, or how long it takes Earth to make one trip around the sun. A solar year lasts 365.24 days, so most calendar years are 365 days long. Every four years, a “leap day” makes up for the extra quarter-days of Earth’s orbit.
But not everyone uses the Gregorian calendar to mark the new year. Some celebrations depend on lunar calendars instead. Lunar calendars are based on the phases of the moon. They don’t match up neatly with the Gregorian calendar.
The Chinese Lunar New Year and the Jewish holiday of Rosh Hashanah are both on lunar calendars. Their dates on the Gregorian calendar change from year to year. However, many lunar calendars include extra months every couple of years. This realigns them with the solar year and keeps the holidays in the same season.