Chinese lunar calendar divides the year into 24 solar terms. Summer is the 7th term and this year it starts (立夏) on May 5th.
Start of Summer marks the transition of seasons. The origin of the term “Start of Summer – 立夏” dates back to 239BC, at the end of the Warring States Period. Since summer is an important season for the harvest, Chinese emperors attached great importance to the start of Summer.
In ancient China people believed that eggs, thanks to their round shape, symbolized a happy life and eating eggs on the day of start of Summer was a prayer for good health. A common saying tells “立夏吃蛋, 石头踩烂”: eating eggs on the day of start of Summer gives you strength.
Source: China Culture