Did you know that the seven days of the week were named after the seven classical planets—the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn?

Sun

Moon

Mars

Mercury

Jupiter

Venus

Saturn

A brief history

The tradition appears to have began with the Romans, who named the days after their gods. However, when this concept reached China, the planets were paired with the Five Phases (五行, Wǔxíng) of Chinese philosophy. Each phase—Wood (木, mù), Fire (火, huǒ), Earth (土, tǔ), Metal (金, jīn), and Water (水, shuǐ)—were associated with a planet based based on its characteristics. For instance, Mars (火星, Huǒxīng), appearing red, was linked to fire (火, huǒ). Venus (金星, Jīnxīng), often perceived as a gray/yellow, was associated with metal or gold (金, jīn). Similarly, Mercury (水星, Shuǐxīng) represented water (水, shuǐ), Jupiter (木星, Mùxīng) symbolized wood (木, mù), and Saturn (土星, Tǔxīng) corresponded to earth (土, tǔ). The Sun (日, rì) and Moon (月, yuè) were integrated into this system.

A lot of history later, while the Chinese system has since evolved, the Japanese words for the days of the week retain this celestial-elemental connection.

Days of the week

In Japanese, the days of the week have a planetary relavent prefix for e.g. 日 (sun), followed by 曜日・ようび, which means day of the week.

Sunday

In Japan, the first day of the week is 「日曜日(にちようび)」 Sunday. This is seemingly because it was believed that the sun should come before the moon.

昨日(きのう)は　日曜日(にちようび)　でした。

Yesterday was Sunday.

Monday

The second day of the week in Japan is 「月曜日(げつようび)」 Monday. The kanji 「月」 represents the moon, which follows the sun.

今日(きょう)は　月曜日(げつようび)　です。

Today is Monday.

Tuesday

The third day of the week is 「火曜日(かようび)」 Tuesday. The kanji 「火」 means fire and is associated with Mars 「火星(かせい)」(kasei), "the red planet".

明日(あした)は　火曜日(かようび)　です。

Tomorrow is Tuesday.

Wednesday

In Japan, the fourth day of the week is 「水曜日(すいようび)」 Wednesday. The kanji 「水」 means water and corresponds to Mercury 「水星(すいせい)」(suisei).

水曜日(すいようび)は　休み(やすみ)　です。

I'm off on Wednesday.

Thursday

The fifth day of the week is 「木曜日(もくようび)」 Thursday. The kanji 「木」 means wood and represents Jupiter「木星(もくせい)」(mokusei).

来週(らいしゅう)の木曜日(もくようび)店(みせ)に　行(い)きます。

Next Thursday I will go to the store.

Friday

The sixth day of the week is 「金曜日(きんようび)」 Friday. The kanji 「金」 means metal or gold and is tied to Venus 「金星(きんせい)」(kinsei). For a deeper look at Friday, click here.

先週(せんしゅう)の金曜日(きんようび)に　カフェに　行(い)きました。

Last Friday i went to a cafe.

Saturday

The final day of the Japanese week is 「土曜日(どようび)」 Saturday. The kanji 「土」 means earth and corresponds to Saturn　「土星(どせい)」(dosei).

私(わたし)の　誕生日(たんじょうび)は　土曜日(どようび)　です。

It's my birthday on Saturday.

Kanji Breakdown of 曜日(ようび)

曜 has meanings of "shining object", and historically used for the classical planets. It’s not used much on its own in common words.

日: has the meanings of "day" or "Sun," referring to the passage of time, the daily cycle, or the day itself. In this context, it means "day of the week."

Related vocabulary

While learning the days of the week, it can be helpful to also know some other key time-related words that often come up in conversations about schedules and plans. These include:

• 今日　きょう　(kyou) - today
• 明日　あした　(ashita) - tomorrow
• 昨日　きのう　(kinou) - yesterday
• 来週　らいしゅう　(raishuu) - next week
• 先週　せんしゅう　(senshuu) - last week

These words make it easy to talk about when something happens, whether you're chatting about your plans for tomorrow, what you did last week, or what’s happening today! Adding more context will make your Japanese feel more natural, and combining applicable vocabulary together will improve your retention and familiarity of the words.

Conclusion

If you haven’t already, check out the post on days of the week to pair these time expressions with specific days like Monday or Friday. Mastering both will help you build natural-sounding sentences in Japanese!