星期三(xīng qī sān) - Wednesday in Mandarin

星期三 (xīngqī sān) is Wednesday, 星期 (xīngqī, "week") plus 三 (sān, "three"), sitting squarely in the middle of the numbered weekday run.

Mandarin speakers do actually call it the midpoint: 星期三是一周的中间 (xīngqī sān shì yì zhōu de zhōngjiān), "Wednesday is the middle of the week", using 中间 (zhōngjiān, "middle") the same way English reaches for "hump day".

As a time word it behaves exactly like the other weekdays, sliding in front of the verb with no preposition: 星期三下午我有空 (xīngqī sān xiàwǔ wǒ yǒu kòng), "I am free on Wednesday afternoon", stacking straight onto 下午 (xiàwǔ, "afternoon") to narrow the time further.