This goes back to the days when the working week was 5½ days. Most institutions worked Mon - Fri and Saturday morning. Shops were open all day Saturday so to compensate they closed at lunchtime mid-week. It wasn't always Wednesday though, in some areas 'early closing day' might be a Tuesday or a Thursday. When the 5 day working week came in shops introduced the system of a morning off, whereby one day of the week you would not start work till lunchtime, then have Wednesday afternoon off, to ensure the shop was fully staffed on Saturday. If you were lucky enough to have Wednesday morning as your half day you were quids in, as you had a whole day off.