Cast off what is obsolete or wrong, or not right for the coming times. Take away a skin or other outside that hinders life. Not all skins can grow, the ones who cannot have to be renewed when the time has come.
It has to be done at the right time, when the old is on the point of breaking, and the new has the possibility to grow. Like the snake sheds his skin at the exact right moment. Sì-day is the day of the snake, the best possible day for skinning.
GE: The character represents probably an animal skin, mounted on a frame of tied-together branches. Part of it (1) might be rams horns. GE: leather; hide, dismiss; expel, perilous.
The trigrams: Fire below Heaven:
In the middle of the lake is fire: skinning. A noble one, by regulating the times, makes the seasons clear.
Time is change, without change there is no time, but there is no life either. Everything needs to be renewed from time to time or it will become obsolete and die away.