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"?