12月21日在中國傳統曆法中具有相當特殊的意義，這一天不僅是二十四節氣之一的「冬至」，同時也被民間稱為「天地合德四喜日」。所謂「天地合德」，源自古代曆法與陰陽五行觀念，指的是天地之氣在特定時刻達到協調與相合的狀態；而「四喜日」則是因為這一天同時符合四個被視為大吉的吉辰條件，分別是天赦日、天願日、天德合與月德合。

這種情況並非年年出現，必須在冬至當天，四大吉辰同時重合，才能形成如此罕見的曆法組合。由於條件嚴苛，發生頻率極低，因此在民間被視為極具祥瑞意味的日子。傳統觀念認為，這類日子象徵天地運行順暢、陰陽調和，有利於消災解厄、祈福納吉，也適合進行祭祖、祈福、行善或立下重要心願。

在部分民俗說法中，冬至本身就有「一陽初生」的含義，代表寒極而陽生，是一年中能量轉折的重要節點；當冬至又與多重吉日重疊時，更被解讀為「吉上加吉」，寓意新的循環即將展開。也正因如此，「天地合德四喜日」往往在民間引發關注，被視為難得的好日子，寄託人們對平安、順遂與未來轉運的期待。

December 21 holds special significance in the traditional Chinese calendar. It is not only the Winter Solstice, one of the 24 solar terms, but is also referred to in folk tradition as the “Heaven-and-Earth in Harmony Fourfold Auspicious Day.” The idea of “heaven and earth in harmony” comes from ancient calendrical theory and the concepts of yin–yang and the Five Elements, describing a moment when the forces of heaven and earth are believed to be in balance and alignment. The term “fourfold auspicious” refers to the rare coincidence of four highly favorable days occurring at the same time: Tian She Day (Day of Heavenly Pardon), Tian Yuan Day (Day of Heavenly Wish), Tian De Harmony, and Yue De Harmony.

This combination does not occur every year. It requires the Winter Solstice to fall exactly on a date when all four auspicious conditions overlap, making it an exceptionally rare calendrical phenomenon. Because of this rarity, it is widely regarded in folk belief as a highly fortunate and propitious day. Traditionally, such a day is thought to symbolize smooth cosmic order and balanced yin and yang, making it especially suitable for dispelling misfortune, praying for blessings, honoring ancestors, performing charitable acts, or making important wishes.

In many traditional interpretations, the Winter Solstice itself already carries the meaning of “the birth of yang energy”, marking the moment when extreme cold gives way to renewal and growth. When the Winter Solstice coincides with multiple auspicious signs, it is seen as “good fortune upon good fortune,” symbolizing the beginning of a new cycle. For this reason, a “Heaven-and-Earth in Harmony Fourfold Auspicious Day” often attracts public attention and embodies people’s hopes for peace, smooth progress, and a positive turning point in the future.