《淮南子·天文训》
八月秋分和二月春分时，阴阳二气不盛不衰平衡相处则昼夜相等，所以说阴气阳气便集合在大门。
八月、二月，阴阳气均，日夜分平，故曰刑德合门。
At the autumn equinox in August and the spring equinox in February, the yin and yang energies are neither strong nor weak, and they exist in balance, so day and night are equal, so it is said that the yin energy and yang energy gather at the front door.