The traditional East Asian calendars divide a year into 24 solar terms. Vernal equinox (春分Chūnfēn) 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°. It more often refers in particular to the day when the Sun is exactly at the celestial longitude of 0°. In the Gregorian calendar, it usually begins around 20 March and ends around 4 April (5 April East Asia time).
根据中国的日历把一年分为了24个节气。春分是春季九十天的中分点。二十四节气之一。太阳位于黄经0°。在时间周期上是指太阳位于黄经０°和１５°之间的位置，大约是3月20日至3月22日之间。但在实务上通常特指太阳真正位于黄经０°的那天：3月20日或3月21日。

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:
中国古代将春分分为三候：“一候元鸟至；二候雷乃发声；三候始电。”

First pentad: 'The dark birds arrive'. 'Dark bird' in this case refers to swallows, which are also making their northward migration.
玄鸟至：玄鸟，燕也，高诱曰：“春分而来，秋分而去也。”

Second pentad: 'Thunder sounds', referring to the onset of spring thunderstorms.
雷乃发声：阴阳相薄为雷，至此，四阳渐盛，犹有阴焉，则相薄乃发声矣；乃者，《韵会》曰：“象气出之难也。”注疏曰：“发，犹出也。”

Last pentad: 'Lightning begins'. This refers to thunderstorms as well, but also to the gradual lengthening of daytime, and the prevalence of sunlight.
始电：电，阳光也；四阳盛长，值气泄时而光生焉。故《历解》曰：“凡声，阳也；光，亦阳也。《易》曰：“雷电合而章。”《公羊传》又曰：“电者，雷光。”是也，徐氏曰：“雷阳，电阴。”非也。盖盛夏无雷之时，电亦有之，可见矣。

 

Customs in Vernal equinox春分习俗
The 24 Solar Terms 24节气