You thought that the spirits and the deities are all the time present in the temples?! Noo... They also have their day off and need to rest.
According to the Daoist tradition, on every Wu 戊 day (the days with Yang Earth heavenly stem in Chinese calendar) the spirits are gone, so you don't need to pray to them, chant and read the scriptures, perform rituals, beat the bell and the drum, burn incense... This is called "No rituals on Wu days" 戊日不朝. In Chinese calendar the Wu days happen at every ten days.
If a Daoist priest works on a Wu (Yang Earth) day, it is regarded as a sin. And if he follows the rule to not perform any ritual on that day, then "the benefits and virtues have no boundaries" 功德无量. That's why if you visit a Daoist temple on such a day, on the door of the main hall most probably you will see a sign "Today is a Wu day" or "Forbidden Wu Day", indicating that there are no rituals.
The idea of "the forbidden Wu day" originates from an old legend from Song Dynasty, according to which on every Wu day the Old Supreme Elderly Lord 太上老君 (one of the three highest deities in the Daoist pantheon) is busy recording the people's destinies in the heavenly archives - who is going to be born, who - to die, who - to be married, etc.
There are many ancient texts which mention the "forbidden Wu Day". Even the famous "Bao Pu Zi"《抱朴子》comments:
"When the Heaven and Earth encounter Wu day, they don't move;
When the army encounters Wu day, there are injuries;
When the snake encounters Wu day, it does not go in [the Earth],
When a swallow encounters Wu day, it does not collect mud pellets to make a nest".
And if the deities, the spirits and all creatures avoid the Wu days, so do the Daoists as well. It's a day off, a good opportunity for the priests and everyone to rest from the temple work and the visitors. A time to relax!
The Metal Monkey Day 庚申日