The traditional Chinese calendar divides a year into 24 solar terms.^[1] Chūnfēn, Shunbun, Chunbun, or Xuân phân is the 4th solar term. It begins when the Sun reaches the celestial longitude of 0° and ends when it reaches the longitude of 15°. In the Gregorian calendar, it usually begins around 20 March and ends around 4 April (5 April East Asia time). It more often refers in particular to the day when the Sun is exactly at the celestial longitude of 0°.

Each solar term can be divided into 3 pentads (候). They are: first pentad (初候), second pentad (次候) and last pentad (末候). Pentads in Chunfen include:

• First pentad: 玄鳥至, 'The dark birds arrive'. 'Dark bird' in this case refers to swallows, which are also making their northward migration.^[2]
• Second pentad: 雷乃發聲, 'Thunder sounds', referring to the onset of spring thunderstorms.^[2]
• Last pentad: 始電, 'Lightning begins'. This refers to thunderstorms as well, but also to the gradual lengthening of daytime, and the prevalence of sunlight.^[2]

A pentad as follows was referred to Japanese traditional calendar presented in a smaller, easy to use, format.

• First pentad: Suzume hajimete sukuu (雀始巣), 'Sparrow begins holding a nest'.^[3]
• Second pentad: Sakura hajimete hiraku (桜始開), 'Cherry blossoms open for the first time'.^[3]
• Last pentad: Kaminari sunawachi koeo hassu (雷乃発声), 'Distant thunder start to sound'.^[3]

• Equinox