When combined, 星期 (xīngqī) literally translates to “star period” or “star cycle.” This name comes from the ancient system of tracking a seven-day week based on the seven classical celestial bodies (the Sun, the Moon, and the five visible planets). Adding 日 (rì), the character for “sun,” makes 星期日 the “Sun's day” of the star cycle, perfectly mirroring the etymology of the English word “Sunday.”
The modern seven-day week is a relatively recent adoption in China, and its naming reflects a practical, systematic approach rather than a mythological one. Unlike Western languages that name days after Norse or Roman gods (e.g., Thursday for Thor, Saturday for Saturn), the Chinese system is mostly numerical. Monday is 星期一 (Week Day 1), Tuesday is 星期二 (Week Day 2), and so on. Sunday is the main exception to this numerical pattern. Instead of “星期七” (Week Day 7), it is called 星期日 (Sun Day) or 星期天 (Sky/Heaven Day). This highlights a parallel with the West's “Sunday.” While Sunday is a day of rest and leisure in modern China, part of the standard two-day weekend (周末, zhōumò), it doesn't carry the same traditional religious weight it does in Christian-majority cultures. For most Chinese people, it's a secular day for family gatherings, shopping, entertainment, or catching up on studies. It's simply the end of the weekend before the work week begins again on 星期一 (Monday).
Understanding when to use 星期日 versus its alternatives is key to sounding fluent.