Leaf on vellum from a manuscript Book of Hours.

Every Book of Hours started with a calendar : the purpose of it was the same as today - look up a birthday! Namely that of the saint of that day, whose birthday was celebrated. ( Did you know that the saints' birthdays were really their dying days? The belief was that on that day they were born in Heaven!).
The important days are given in red, ?red-letter-days ?, the rest in blue. At the top we see ?KL ? for kalend, the Latin word for calendar and Septembre ha[bet] xxx Jour. La lune xxix. [ September has 30 days, the moon 29.]
In the margins we find decorations of acanthus leaves and flowers in red, blue, green and gold.

Reference: pw47