The Gregorian calendar officially began to be used in 1896. The dates of older important Korean events are still based on the old calendar. The most important date in Korea today is Seollal, the first day of the Korean calendar's New Year. Other important events include Daeboreum (the first full moon phase, also referred to as "Boreumdaal"), Dano (spring) and Chuseok (autumn), and Samjinnal (beginning of spring). Other minor events based on the old Korean calendar include Yudu (summer), and Chilseok (monsoon season).