THE STANDARD LUNISOLAR CALENDAR & THE 24 SOLAR TERMS

The lunar calendar follows the waxing and waning of the moon: each month begins at the dark moon, the first of the month, runs to the full moon on the fifteenth, then wanes again. Vietnamese people use it for death anniversaries, feast days, and auspicious and inauspicious days. My calendar works out the lunar date by the method of Hồ Ngọc Đức, referred to Vietnam time, so it matches what people actually keep.

Our calendar is in fact lunisolar: it follows the moon to set the months and the sun to set the seasons through the twenty-four solar terms. Because a lunar year is about eleven days shorter than a solar year, an intercalary month must be added every few years so that the seasons do not drift.

Because a lunar year is shorter than a solar year, and left alone, Tết would drift into the summer. The ancients set the rule: a lunar month that contains no major term is taken as the intercalary month, and the month containing the Winter Solstice 冬至 is always the eleventh. Every nineteen years there are seven intercalary months, which is the Metonic cycle.

The Stem and Branch is the Eastern way of recording time, pairing the ten Heavenly Stems 天干 (Giáp 甲, Ất 乙, Bính 丙, Đinh 丁, Mậu 戊, Kỷ 己, Canh 庚, Tân 辛, Nhâm 壬, Quý 癸) with the twelve Earthly Branches 地支 (Tý 子, Sửu 丑, Dần 寅, Mão 卯, Thìn 辰, Tỵ 巳, Ngọ 午, Mùi 未, Thân 申, Dậu 酉, Tuất 戌, Hợi 亥) into a cycle of sixty. Every year, month, day and hour has its own Stem and Branch pair.

The solar terms are twenty-four marks dividing the sun's apparent path through the year evenly, fifteen degrees of ecliptic apart, answering to the turning of the seasons: the Beginning of Spring 立春, the Spring Equinox 春分, the Summer Solstice 夏至, the Winter Solstice 冬至. The solar terms decide the month in the Four Pillars 八字 and the farmer's season. I work them out by the astronomical algorithm of Meeus, so they are exact to the minute.

Each day has twelve hours, six of them Hoàng Đạo 黃道 hours guarded by auspicious spirits such as Thanh Long 青龍, Minh Đường 明堂 and Kim Quỹ 金匱, and six Hắc Đạo 黑道 hours governed by inauspicious ones. Anything weighty should be done in an auspicious hour. The auspicious hours are set by the Earthly Branch of the day.

The Officer is one of twelve stars taking the day in turn: Kiến 建, Trừ 除, Mãn 滿, Bình 平, Định 定, Chấp 執, Phá 破, Nguy 危, Thành 成, Thu 收, Khai 開, Bế 閉. Each Officer suits and forbids different undertakings; for instance Thành 成 suits weddings and openings, while Phá 破 forbids anything large. A day whose Earthly Branch matches the Branch of the solar term month is a Kiến 建 day.

They are twenty-eight constellations in the sky, taking one day each in turn, divided into auspicious and inauspicious mansions. Each has its phase, its animal and its own things to avoid; for instance the mansion Vĩ 尾 belongs to Fire, has the tiger as its animal, and suits weddings and building.

The Nạp Âm 納音 is the phase assigned to each Stem and Branch pair in the sexagenary cycle 六十花甲; for instance Giáp Tý 甲子 and Ất Sửu 乙丑 are Hải Trung Kim 海中金, and Canh Ngọ 庚午 is Lộ Bàng Thổ 路傍土. The Nạp Âm is used to read a person's destiny and to work out the generating and controlling relations between years.

The Ngọc Hạp Thông Thư 玉匣通書 is a classical work listing a great many auspicious spirits and inauspicious Sha placed by month and by day: Thiên Đức 天德, Ngũ Phú 五富 and Phúc Sinh 福生 are auspicious, while Kiếp Sát 劫煞, Đại Hao 大耗 and Không Phòng 空房 are inauspicious. Reading a day means weighing which auspicious and inauspicious stars shine on it, to choose or to avoid a task.

Each cycle of sixty Stem and Branch pairs divides into six sets of ten days, and the two Earthly Branches left over with no Stem to pair with are the Void 空亡 of that set. A day or an hour falling in the Void is held to be empty, and what is sought there is hard to attain.

These are days avoided by the lunar calendar. The Tam Nương 三娘 are the 3rd, 7th, 13th, 18th, 22nd and 27th. The Nguyệt Kỵ 月忌 are the 5th, 14th and 23rd. The Dương Công Kỵ Nhật 楊公忌日 are thirteen days set down by Dương Công 楊公. All are avoided for beginning anything large.

These are the directions for setting out on a given day. Hỷ Thần 喜神 is the direction that meets good fortune and joy, Tài Thần 財神 the direction for seeking wealth, and Hạc Thần 鶴神 an inauspicious direction to avoid. They are set by the Heavenly Stem of the day.

They are six stars, Đại An 大安, Lưu Niên 留連, Tốc Hỷ 速喜, Xích Khẩu 赤口, Tiểu Cát 小吉 and Không Vong 空亡, taking their turn by lunar month and day, used to judge quickly whether a journey or a venture for gain will go well. Tốc Hỷ 速喜 and Đại An 大安 are good; Xích Khẩu 赤口 and Không Vong 空亡 are bad.

This is a way of choosing an hour to travel, handed down from Lý Thuần Phong 李淳風 of the Tang dynasty. It divides the hours of the day into six auspicious and inauspicious hexagrams that turn with the lunar day, each with its own reading on gain, meetings and news.

They are the admonitions of Bành Tổ 彭祖 on what to avoid according to the Stem and the Branch of the day; for instance, on a Giáp 甲 day do not open a store, and on a Hợi 亥 day do not hold a wedding. Each day has one line by its Stem and one by its Branch.

Flying Star Feng Shui 玄空飛星 is the method of placing nine stars into nine palaces by year, month, day and hour. Ngũ Hoàng 五黃 is the great Sha star: do not break ground in the direction where it lands. Tam Sát 三煞, Tuế Phá 歲破 and Thái Tuế 太歲 are the inauspicious directions of the year, also to be avoided when building.

The ancients divided each solar term into three pentads of about five days each, seventy-two in the year, each recording a sign of the season turning in nature: a flower opening, cicadas calling, the wild geese returning. It is a way of living in step with heaven and earth.

The tide is the rhythm of the water rising and falling. Near the first of the month and the full moon, the sun and the moon pull together and the tide runs strong: a spring tide. Near the quarters the tide is weak: a neap tide. Fishermen put to sea by the tide.

These are the adverse relations between the Earthly Branches. The Six Clashes 六沖 are two Branches opposite one another, such as Tý 子 with Ngọ 午. The Three Punishments 三刑, the Six Harms 六害 and mutual Breaking 相破 are milder forms of conflict. On a day that clashes with a person's year, that person should avoid anything weighty.