(idiomatic) an auspicious day in the lunar calendar on which a certain activity such as marriage is favoured.
Etymology
This traces back to the days when people superstitiously believed that the point the Earth is at on its orbit around the sun has an effect on how auspicious a given day in the lunar calendar would turn out. This would influence when people would go travelling, get married, open a business and etc. Literally: "the Earth orbit around the sun and auspicious day"