What does the lunar calendar mean?

There are ten heavenly stems and twelve earthly branches, so every time a branch is allocated, there must be two earthly branches that are "empty", which is called "empty death".

Simple:

For example: Jia Zi, Yi Chou, Bing Yin, Ding Mao, Chen Wu, Keith, Geng Wu, Xin Wei, Ren Shen, Gui You, Kong Xu and Kong Hai. The last two do not match and can only be empty.