Japanese and Korean Both Name Days After the Sun, Moon, and Planets
Japanese weekday names (月火水木金土日) use a system based on the sun, the moon, and five visible planets — a tradition Korean shares as well, naming its days the exact same way with the same characters. Western weekday names have a similar origin from a different mythology: English and other European languages trace many day names back to Roman or Norse gods tied to the sun, moon, and planets, such as the sun (Sunday), the moon (Monday), or Saturn (Saturday). East and West arrived at the same basic idea — naming days after the sky — independently.
Vietnamese and Mandarin Both Number Their Days
Vietnamese counts its weekdays rather than naming them individually: Monday is literally "day two" (Thứ Hai), Tuesday is "day three" (Thứ Ba), and so on, counting from Sunday (Chủ Nhật, "the main day"). Mandarin Chinese follows similar logic — 星期一 (Monday) through 星期六 (Saturday) simply attach a number to "week" (星期), with only Sunday, 星期日, breaking the pattern. Once you know the numbers one through six, you already know most of the weekday names in both languages.
Frequently Asked Questions
Why do some calendars start the week on Sunday instead of Monday?
It comes down to differing cultural and religious traditions for what counts as the start of a week. Some countries, including the United States, treat Sunday as the first day of the week, while the ISO international standard used across much of the world — and the default in most calendar apps — treats Monday as the first day.
Is there another common way to say "Sunday" in Mandarin?
Yes — alongside the more formal 星期日 (xīngqīrì), 星期天 (xīngqītiān) is very commonly used in everyday speech. Both mean exactly the same thing.