Why are calendar grids always exactly 5 rows high?


Where, when, and how was it decided that a calendar grid should always have exactly five rows, even where logic would seem to dictate six rows?

Why is this convention used even on wall calendars which clearly have space for a sixth row, even if this would mean decreasing space used for a "notes" section or pithy quotes or some such? Note that only a partial sixth row is ever needed.

Even if for some reason it is forbidden to use a sixth row, why use "23/30" and/or "24/31", thus cramming five characters into a single cell? Why not instead use "1/8" and/or "2/9"?

Proportional vs. monospaced characters, and a calendar dilemma


I am in the process of designing a wall calendar, with one page per week. (I am using CorelDRAW, if you must know.) I am using the fonts DIN Engschrift and DIN Mittelschrift, mainly the latter.
It being a calendar, numerals are the most important characters. I am going for function rather than beauty, but still, I don't want an ugly calendar. This is why I chose a road sign font: I want to be able to read the date from across the room without my eyeglasses! Also, monospaced figures are a must.
I really do not like the way that the days of the week look like they are going to turn out, though. Please see the picture. It is a screenshot of me testing several fonts in a spreadsheet program. No wonder Wednesday is nicknamed "hump day"!