The literal translation of the title of this movie is: Sunny Day.
This movie is the typical political propaganda of mid Cultural Revolution era, and follow the standard format of eulogizing the Cultural Revolution, and denouncing Field Marshal Biao Lin, but without any bashing on Xiao-Ping Deng, who later became the paramount leader of China and started the reform in China, resulting in the rapid Chinese economic expansion today.
The reason Deng was spared temporarily was because by the middle of the Cultural Revolution, he was still in the labor camp in the remote region of Jiang-Xi province in southern China, and Mao had not named him deputy premier yet. Deng bashing would appear in the later political propaganda films of the Cultural Revolution era.