calendar-month-edges

calendar-month-edges is a variable defined in `calendar.el'.
Its value is

((0 0 5 24 28) (1 28 30 49 53) (2 53 55 74 78))

Documentation:
Alist of month edge columns.
Each element has the form (N LEFT FIRST LAST RIGHT), where
LEFT is the leftmost column associated with month segment N,
FIRST and LAST are the first and last columns with day digits in,
and LAST is the rightmost column.