When the ancients (of course, literati and distinguished people) said farewell, they often went to the "Ten Miles Pavilion" to drink farewell wine, so there is a saying of "farewell tour". (Farewell: When relatives and friends want to travel far away, they prepare a banquet to see them off as a way to express their blessing and farewell.
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It is customary to have this farewell meal at lunch. For literati and elegant people, one is to send each other to a pavilion in the suburbs to say goodbye, and the other is to recite poems and drink while saying goodbye. It takes a long time to say goodbye. Wait until the red sun turns to the west, then bid farewell - this makes it even more sincere!
Therefore, when it is time to officially "shake hands and say goodbye", it is often "sunset"!