What does "three-five commendation" mean?

Commending on March 5th refers to commending a group of people with Lei Feng spirit.

Because the anniversary of learning from Lei Feng is March 5th every year. On March 5th, 1963, President Mao Zedong's inscription "Learn from Comrade Lei Feng" was published in People's Daily. Since then, learning from Lei Feng has been widely carried out throughout the country.

Lei Feng, formerly known as Lei Zhengxing. A certain unit of China People's Liberation Army transports re-elected soldiers and squad leaders. 1962 August 15 died in the line of duty in Wanghua District, Fushun City, Liaoning Province, at the age of 22.