Vietnamese commonly choose auspicious days for important events, especially for marriage rites. Tradition holds that a couple's future depends on "age compatibility" of the would-be spouses and on a propitious day and hour for the engagement and wedding ceremonies. In the old days, both sets of parents consulted a Confucian scholar or a professional soothsayer to choose lucky days and hours. Sometimes, at the urging of a soothsayer, a couple would delay the wedding for several months.
Oriental astrology is extremely complicated. Simply put, the Vietnamese system for reckoning time names each month of the twelve-month cycle after an animal, beginning with the Tiger. The Chinese and Vietnamese calendars are the same except that the Vietnamese replace the hare with the cat.
The calendar divides each day into twelve two-hour segments, each named for an animal. Chinese and Vietnamese say Buddha decided the order of the animals thousands of years ago by inviting all the animals to a New Year's gathering. Buddha named the months after the twelve animals that showed up, listing them in the order of arrival. However, the truth is that the animal names in the Oriental zodiac existed long before the birth of Buddha.
Auspicious days vary according to the month:
- First month (the Tiger) and seventh month (the Monkey): all the days of the Rat.
- Second month (the Cat) and eighth month (the Rooster): all the days of the Tiger.
- Third month (the Dragon) and ninth month (the Dog): all the days of the Dragon.
- Fourth month (the Snake) and tenth month (the Pig): all the days of the Horse.
- Fifth month (the Horse) and eleventh month (the Rat): all the days of the Monkey.
- Sixth month (the Goat) and twelfth month (the buffalo): all the days of the Dog.
After determining an auspicious day, the soothsayer uses the same principle to discern the auspicious double-hour for that day. For example, the auspicious time on the day of the Tiger or Monkey is double-hour of the Rat (11 pm — 1 am). The auspicious time on the day of the Cat and the Rooster is that of the Tiger (3-5 am.)
Some people also take into account the constellations governing the day. They will avoid days that might seem auspicious for other events if those dates fall during marriage-adverse constellations, such as "Lonely Room," "Nest Quitting," and "Widowhood."
Nowadays, fewer and fewer people can decipher the ancient calendar because it is written in nom ideograms. As a result, most Vietnamese rely on simplified translations published in modern script. However, these versions are far removed from the ancient science of the stars.