The traditional Chinese calendar divides a year into 24 solar terms.
Lìdōng, Rittō, Ipdong, or Lập đông (Chinese and Japanese: 立冬; pinyin: lìdōng; rōmaji: rittō; Korean: 입동; romaja: ipdong; Vietnamese: lập đông; lit.
It begins when the Sun reaches the celestial longitude of 225° and ends when it reaches the longitude of 240°.
It more often refers in particular to the day when the Sun is exactly at the celestial longitude of 225°.
[1] Lidong signifies the beginning of winter in East Asian cultures.