In the early summer of 744 AD, the Tang Dynasty's two greatest poets, Li Bai, then 44, and Du Fu, then 33, met in Luoyang and became friends at first sight.
After living in Luoyang for a time, Li Bai married in Liang Garden, and Du Fu traveled specially to offer his congratulations.
The following spring, they traveled together to Qizhou to visit Li Yong, the governor of Beihai, who hosted a banquet in their honor and invited them to share their aspirations.
Du Fu said, "Every day I think about how to diligently serve the court and the people, living up to my father's expectations." Li Bai said, "I will use the crescent moon as a hook, the rainbow as a line, and the unjust villain as bait, to fish for giant turtles in the vast sea, doing my part for the people of the world."
Li Yong admired both of them and invited them to stay longer, so they spent the entire summer in Qizhou.
In late June, Li Bai bid farewell to Du Fu and set off for Yanzhou. The following autumn, Du Fu arrived in Yanzhou as well, and the two drank freely and composed poetry together. By day they roamed hand in hand, and by night they shared a bed, deepening their friendship even further.
Soon after, Du Fu was heading west to Chang'an, while Li Bai planned to revisit the Jiangdong region, so the two parted reluctantly.
Du Fu arrived in Chang'an hoping to secure an official post, but his wish remained unfulfilled. In his loneliness, he often missed Li Bai and wrote a poem titled "Thinking of Li Bai on a Spring Day." The final four lines read: "Spring trees north of the Wei River, / Evening clouds east of the River. / When can we share a cup of wine, / And discuss fine literature again?"
While living in Chang'an by the Wei River, the poet Du Fu likened himself to an ancient tree in spring and compared his wandering friend Li Bai, who was roaming the Jiangdong region, to the evening clouds of spring, vividly expressing his deep longing for his distant companion.
Later, the idiom "Evening Clouds and Spring Trees" came to describe longing for a distant friend.
Source: Du Fu (Tang Dynasty), "Thinking of Li Bai on a Spring Day"
Meaning of the Idiom: Later, the Chinese idiom "暮云春树" came to describe how longing for a distant friend.