"一年之计在于春，一日之计在于晨" (Yì nián zhī jì zàiyú chūn, yí rì zhī jì zàiyú chén. A year's plan starts with spring; a day's plan starts with early morning) is a popular Chinese proverb that shows how Chinese people think of the changing of the seasons during a year and how they relate to one's daily life.
"一年之计在于春 (yì nián zhī jì zàiyú chūn. A year's plan starts with spring) was summed up by the farmers during their cultivation of crops. It emphasizes the importance of spring out of the four seasons and that a good harvest depends on the spring sowing.
"一日之计在于晨" (yí rì zhī jì zàiyú chén, A day's plan starts with early morning) refers to the best time of a day being the early morning, and that it is good to make a whole day's plan early in the morning. What's more, in Chinese language, teenagers are compared with the spring, since they are full of hopes and dreams for their lives ahead of them.
That's why the Chinese word "青春" (qīngchūn, youthful) which contains the character "春" (chūn, spring) is used to describe the young people who are at the spring season of their life.