Strictly, midweek is Wednesday — the middle day of a Monday-to-Sunday week. Loosely, it covers Tuesday to Thursday, the stretch with no weekend at either end. The page above gives the strict answer, shows how much of the week has actually elapsed, and counts down to Wednesday midday.
The midpoint depends on where the week starts
Under ISO 8601 the week runs Monday to Sunday, putting the midpoint on Wednesday. Under the Sunday-start convention common in the United States, the middle day is Wednesday too — but the week's halfway *moment* lands a day earlier. Which convention your calendar uses is covered in Monday vs Sunday week start.
Why Wednesday earned a nickname
No other weekday has one. Wednesday got "hump day" because it is the point where the week stops being ahead of you and starts being behind you — the crest of the climb. Where the phrase came from is in where hump day comes from.