Quick Reference

Meaning & Definition

Among the seven days of the week in Japanese, Thursday stands out as the only day whose name begins with the kanji for tree or wood — 木 (moku). This is no coincidence: the Japanese names for the days of the week were inherited from a system rooted in ancient Chinese cosmology, where the five classical elements and the visible planets of the solar system were mapped onto each day. Thursday’s identity as 木曜日 (mokuyoubi) traces directly to Jupiter — the planet the ancient Chinese associated with the element of Wood.

木曜日 (mokuyoubi) means Thursday. It is a compound of three kanji: 木 (moku, wood/tree), 曜 (you, day of the week), and 日 (bi, day/sun). In everyday speech, Japanese speakers often shorten it to 木曜 (mokuyou) in casual contexts — for example, mokuyou no kaigi (Thursday meeting) rather than the full mokuyoubi no kaigi. The word carries no inherently formal or informal register on its own; its tone is determined by the surrounding sentence. Unlike English, where Thursday is culturally linked to Norse mythology (Thor’s day), 木曜日 carries the weight of East Asian astronomical tradition, specifically the association with the planet Jupiter (木星, mokusei).

How to Use It

A common point of confusion for learners is distinguishing 木曜日 (mokuyoubi, Thursday) from 木曜 (mokuyou, the short form) and from 木 (ki or moku, tree/wood). Note that when 木 is read as part of 木曜日, it is pronounced moku, not ki — the on-reading is used in this compound. Also watch out for the difference between 木曜日 (Thursday) and 木星 (mokusei, Jupiter): both start with 木 and share their astronomical connection, but one is a day and the other a planet. In schedules and timetables, Thursday is commonly abbreviated to just 木 (moku), so recognizing the single kanji is essential for reading calendars.

Kanji Breakdown

The three kanji in 木曜日 each carry a distinct origin. 木 depicts a tree: a vertical stroke for the trunk with branches above and roots below, representing the element of Wood in the five-element (gogyou) system. 曜 is a complex character combining the sun radical (日) with components suggesting brightness and cyclical movement — it is used exclusively for the days of the week in modern Japanese. 日 in this context functions as the suffix meaning “day,” though its core meaning is the sun itself, shown as a circle with a line through the center representing the solar disc.

Example Sentences

Cultural Context

The name 木曜日 is inseparable from the ancient Chinese cosmological system known as 五行 (gogyou), the Five Phases or Five Elements: Wood (木), Fire (火), Earth (土), Metal (金), and Water (水). When this system was applied to the days of the week — borrowing from Indian and Hellenistic astronomical traditions — each of the five visible planets was assigned one of these elements. Jupiter, the largest planet in the solar system and one of the brightest objects in the night sky, was paired with Wood (木) because of its perceived life-giving, expansive qualities. Thursday thus became 木曜日, the Day of Wood, with Jupiter named 木星 (mokusei, literally “Wood Star”) in Japanese and Chinese.

This planet-based naming system gives the Japanese week a cosmological symmetry that is absent from English day names. Monday through Saturday in Japanese correspond directly to the Moon (月, getsu), Fire (火, ka), Water (水, sui), Wood (木, moku), Metal (金, kin), and Earth (土, do) — mapping onto the Moon, Mars, Mercury, Jupiter, Venus, and Saturn respectively. Sunday (日曜日, nichiyoubi) represents the Sun. For Japanese learners who also study astronomy or classical East Asian philosophy, recognizing that 木曜日 literally encodes Jupiter’s place in the sky adds a mnemonic layer: Thursday is the day of the great wooden planet.

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