Together, 星期 (xīngqī) means “star cycle,” which became the modern Chinese word for “week.” By adding the number 三 (sān), you are specifying the third day within that cycle: Wednesday.
The primary cultural significance of “星期三” lies not in the day itself, but in the system it represents. Unlike English, where days are named after Norse and Roman gods (e.g., Wednesday is “Woden's Day”), the Chinese system is purely numerical and logical. This reflects a practical approach to language, prioritizing clarity and ease of use over mythological or historical tradition in this context. For a learner, this is a massive advantage. A key cultural difference is the start of the week. In China, the work and school week officially begins on 星期一 (Monday). This is different from the United States, where calendars often display Sunday as the first day of the week. In modern, urban China, Wednesday is sometimes colloquially called “小周末” (xiǎo zhōumò), which means “little weekend.” It marks the midpoint of the workweek, a “hump day” when people start looking forward to the actual weekend.
“星期三” is the standard, universally understood term for Wednesday. However, in daily life, you'll frequently encounter variations.
All three terms are interchangeable in most everyday conversations.