Books of Hours were intended for private, devotional use. The liturgical hours, the backbone of the Book of Hours, sanctify the day—they order it according to devotional practice. Not only was the day accordingly sanctified, but the seasons were also made to harmonize with the Divine. In order to calculate when the feasts of Easter, Nativity, Ascension, as well as Saints' Days occurred, Books of Hours included Calendars. Red ink signified a Feast or Saint's Day; hence the phrase, “red letter day.”

Calendars varied according to regions and practices, since different regions venerated different saints. One way to determine the provenance of a Book of Hours is to examine its Calendar and discover which Saints and Feasts are peculiar to the book.

Besides the liturgical use of the Calendar, it also divided the year into agricultural seasons. Often the borders of the Calendar depict the twelve signs of the Zodiac and twelve monthly labors (most famously illustrated in Très Riches Heures Du Duc De Berry).

Month Zodiac Labor
January Aquarius Feasting
February Pisces Keeping Warm by the Fire
March Aries Pruning Trees or Vines
April Taurus Picking Flower Branches
May Gemini Riding & Hawking
June Cancer Mowing Hay with a Scythe
July Leo Reaping Wheat with a Sickle
August Virgo Harvesting & Threshing Wheat
September Libra Treading Grapes
October Scorpio Plowing and Sowing Seeds
November Sagittarius Thrashing for Acorns
December Capricorn Slaughtering a Fatted Pig