Japanese Days of the Week Meaning: Why Are They Named After Planets?

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When beginners first learn the days of the week in Japanese, the names can look surprisingly simple:

This article explains:

For pronunciation and everyday scheduling expressions, see the complete guide: Days of the Week in Japanese.

Japanese Days of the Week: Meaning at a Glance

What Does 曜日 (Yōbi) Mean?

Every Japanese weekday ends in 曜日 (ようび / yōbi).

In a weekday name, the first kanji tells you the celestial body, while 曜日 identifies it as a weekday.

Are Japanese Weekdays Named After Elements or Planets?

The most accurate answer is: they are named after celestial bodies, but five of those celestial bodies have element-based Japanese names.

The same distinction applies to the other four:

Why Do the Japanese Planet Names Use Fire, Water, Wood, Metal, and Earth?

The five visible planets were connected in East Asian tradition with the 五行 (gogyō), often translated as the Five Phases or Five Elements.

This is why the same kanji can refer to both a familiar material and a planet.

The Meaning and Origin of Each Japanese Weekday

日曜日 (Nichiyōbi): Sunday and the Sun

日 commonly means both “sun” and “day.” In 日曜日, it represents the Sun as one of the seven luminaries.

日曜日は休みです。
Nichiyōbi wa yasumi desu.
Sunday is my day off.

月曜日 (Getsuyōbi): Monday and the Moon

月 can mean both “moon” and “month.” In 月曜日, it refers to the Moon.

月曜日から仕事が始まります。
Getsuyōbi kara shigoto ga hajimarimasu.
Work begins on Monday.

Learn more: Tsuki Meaning — Why 月 Can Mean Both “Moon” and “Month”

火曜日 (Kayōbi): Tuesday and Mars

Mars appears reddish in the night sky and was traditionally associated with fire in the East Asian naming system.

火曜日に病院へ行きます。
Kayōbi ni byōin e ikimasu.
I will go to the hospital on Tuesday.

水曜日 (Suiyōbi): Wednesday and Mercury

水曜日に日本語のクラスがあります。
Suiyōbi ni Nihongo no kurasu ga arimasu.
I have Japanese class on Wednesday.

木曜日 (Mokuyōbi): Thursday and Jupiter

会議は木曜日です。
Kaigi wa mokuyōbi desu.
The meeting is on Thursday.

金曜日 (Kin’yōbi): Friday and Venus

金曜日の夜は友達と食事をします。
Kin’yōbi no yoru wa tomodachi to shokuji o shimasu.
I have dinner with friends on Friday night.

土曜日 (Doyōbi): Saturday and Saturn

土曜日は家でゆっくりします。
Doyōbi wa ie de yukkuri shimasu.
I relax at home on Saturday.

Japanese and English Weekday Names Follow the Same Planetary Pattern

English and Japanese weekday names may look unrelated, but both reflect the same older association between days, celestial bodies, and deities.

Why Are the Days in the Order Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn?

The sequence may look random:

The order was inherited from an ancient planetary-week system. A traditional explanation uses planetary hours.

Ancient astrologers arranged the seven classical celestial bodies in an order based on their apparent movement and supposed distance from Earth:

They then assigned each hour to the next celestial body in this repeating sequence. Because a day has 24 hours, the celestial body ruling the first hour of the next day moves forward by three places.

How Did the Planetary Week Reach Japan?

The seven-day planetary system did not begin in Japan. Versions of the planetary week circulated across multiple cultures in West Asia, the Mediterranean world, India, and China before reaching Japan.

According to the National Astronomical Observatory of Japan’s calendar reference, the weekday system was brought to Japan in the Heian period together with the Buddhist astrological text 宿曜経 (Sukuyōkyō), traditionally associated with Kūkai, also known as Kōbō Daishi.

The seven-day week became more important in daily life after Japan adopted the solar calendar in the Meiji era. Government rules establishing Sunday as a holiday and Saturday as a half-day also helped the weekly schedule become established.

What Is 七曜 (Shichiyō)?

七曜 literally means the seven luminaries.

In older astronomy and astrology, the Sun and Moon were grouped with the five planets visible to the naked eye. These seven moving or prominent celestial bodies formed the basis of the weekday names.

How to Remember the Japanese Weekday Meanings

Step 1: Learn the Sun and Moon First

These match English almost directly.

Step 2: Connect the Five Kanji to Planet Names

Step 3: Add 曜日 to Make a Weekday

Common Beginner Mistakes

Mistake 1: Thinking 火曜日 Is Only “Fire Day”

The literal kanji image is fire, but the historical weekday reference is 火星, Mars.

Mistake 2: Reading 月曜日 as Tsuki-yōbi

月 is read getsu in this compound.

Mistake 3: Pronouncing 曜日 with a Short “O”

The correct romanization is yōbi. Keep the yō slightly long.

Mistake 4: Thinking the Weekday Order Comes from the Five Elements

The sequence 火・水・木・金・土 in the week was inherited from the planetary-week order. It is not the usual ordering of the Five Phases.

Mistake 5: Translating 金曜日 as “Money Day”

金 can mean money in other words, but in 金曜日 it represents 金星, Venus.

Natural Example Sentences Using Japanese Weekdays

今日は月曜日です。
Kyō wa getsuyōbi desu.
Today is Monday.

火曜日と木曜日に日本語を勉強します。
Kayōbi to mokuyōbi ni Nihongo o benkyō shimasu.
I study Japanese on Tuesdays and Thursdays.

水曜日は予定があります。
Suiyōbi wa yotei ga arimasu.
I have plans on Wednesday.

金曜日の夜は空いています。
Kin’yōbi no yoru wa aite imasu.
I am free on Friday night.

土曜日か日曜日に会いませんか。
Doyōbi ka nichiyōbi ni aimasen ka.
Would you like to meet on Saturday or Sunday?

次の月曜日は祝日です。
Tsugi no getsuyōbi wa shukujitsu desu.
Next Monday is a public holiday.

To learn words such as today, tomorrow, and yesterday, see Kyō, Ashita, and Kinō: Today, Tomorrow, and Yesterday in Japanese.

For a closer look at Monday, see Getsuyōbi Meaning: How to Say Monday in Japanese.

FAQ About the Meaning of Japanese Weekday Names

Why are Japanese days of the week named after planets?

Japanese inherited a seven-day system based on the Sun, Moon, and five classical planets. The five planets were known in Japanese as 火星, 水星, 木星, 金星, and 土星, so their first kanji became the weekday names.

Are Japanese weekdays named after the five elements?

They are more precisely named after celestial bodies. However, the five planet names use the Five-Phase kanji 火, 水, 木, 金, and 土, which is why the weekday names look like element names.

What does 曜日 mean?

曜日 (yōbi) means “day of the week.” It is added to 日, 月, 火, 水, 木, 金, or 土 to create a weekday name.

Why is Monday called 月曜日?

月 means moon, and Monday is the day associated with the Moon. English “Monday” also originally means “Moon’s day.”

Why is Tuesday 火曜日?

火 is the first kanji in 火星, the Japanese name for Mars. Tuesday belongs to the day traditionally associated with Mars.

Why is Wednesday 水曜日?

水 is the first kanji in 水星, Mercury. Wednesday is the day associated with Mercury in the planetary-week system.

Why is Thursday 木曜日?

木 is the first kanji in 木星, Jupiter. Thursday corresponds to Jupiter, just as English Thursday is associated with Thor, the Germanic counterpart used for Jupiter’s day.

Why is Friday 金曜日?

金 is the first kanji in 金星, Venus. Friday is the day associated with Venus.

Why is Saturday 土曜日?

土 is the first kanji in 土星, Saturn. Saturday is also visibly connected to Saturn in English.

Did Japanese weekday names come from English?

No. Japanese and English inherited different forms of an older planetary-week tradition. Japanese uses East Asian planet names, while English uses the Sun, Moon, Saturn, and several Germanic deity names.

When did Japan begin using weekday names?

The seven-luminary system reached Japan in the Heian period and appeared in calendars long before the modern weekly schedule. The seven-day rhythm became more important in ordinary life during the Meiji era.

Summary: Why Japanese Weekdays Are Named After Planets

Next, learn the pronunciation, abbreviations, and everyday usage of all seven days in the hub guide: Days of the Week in Japanese.

References

• National Astronomical Observatory of Japan, Calendar Wiki: Names of the Weekdays — Japanese weekday and celestial-body correspondences, plus transmission through 宿曜経.
• Kotobank: 七曜 — definitions of the seven luminaries and their use in weekday names.
• National Diet Library Reference Collaborative Database: The Introduction and Use of Weekdays in Japan — calendrical use and adoption in modern daily life.
• Merriam-Webster: Etymologies for Every Day of the Week — English weekday names and their planetary or mythological correspondences.
• Timeanddate.com: Why Are There Seven Days in a Week? — the planetary-hour explanation for the order of the weekdays.