This is the 6205th most frequent Chinese word.
建军
"建军" means "to establish an army."
In this sentence, '建军' refers to the act of establishing an army.
中国人民解放军于1927年正式建军。
The People's Liberation Army of China was officially established in 1927.
Here, '建军' is used as a modifier for a holiday related to the army's establishment.
建军节是国家纪念中国军队成立的重要日子。
Army Day is a national holiday commemorating the establishment of China's armed forces.
In this context, '建军' signifies the concept of establishing and maintaining a military force.