More specifically, this is used to rate legs in a covert way - the rating system running from Sunday to Saturday, with "Sunday legs" being too thin and "Saturday legs" being too fat - hence "Wednesday legs" (the mid-point of the week) are just right.
N.B. When rating legs it is important to note that no two pairs of wednesday legs will ever be the same - there is no preset size or dimensions, it is simply how the legs are in proportion to the woman.
Man 1: Look at those wednesday's!
Man 2: I dunno, they look more like mid-tuesday's to me...
(Attractive woman walks away, out of earshot)
Man 1: She had some damn nice wednesday legs.