A random thought stumbled into my mind the other day when I realized that Sunday matched the Japanese word for Sunday is “日曜日”. The “日” part means “sun”. At first I thought it was just a coincidence, then I realized that Monday represented the moon, and in Japanese that’s “月曜日” and the “月” part means moon. Something is clearly going on here. Since the idea of seven days in a week doesn’t originate from Asia, the Japanese days of the week probably just followed western convention, which makes sense. Now when it comes to the English days of the week, the names were originally named by the Romans with the names of the sun, the moon, and the five known planets at the time [1].

Day of the week Latin word (English word)
Sunday Solis (sun)
Monday Lunae (moon)
Tuesday Martis (Mars)
Wednesday Mercurii (Mercury)
Thursday Jovis (Jupiter)
Friday Veneris (Venus)
Saturday Saturni (Saturn)

Now on the other side of the world, the Chinese and the Japanese gave names to these planets using their classical elements (the Chinese and Japanese names are the same). They gave the celestial bodies these names:

Celestial body Chinese/Japanese name Corresponding element
The sun
The moon
Mars 火星 Fire
Mercury 水星 Water
Jupiter 木星 Wood
Venus 金星 Gold
Saturn 土星 Earth/soil

So now, when the Japanese went to adopt the western calendar, they took the base meanings of the days of the weeks by their planets, and used their planet names in their place. So Sunday (from the sun) became “日曜日”, Monday (from the moon) became “月曜日” and so forth. Like this:

Celestial body English day of the week Japanese day of the week
Sun Sunday 日曜日
Moon Monday 月曜日
Mars Tuesday 火曜日
Mercury Wednesday 水曜日
Jupiter Thursday 木曜日
Venus Friday 金曜日
Saturn Saturday 土曜日

Cool, huh?