This article accompanies the fable
Bloodletting, for example: if the Moon was in the Zodiac sign ruling the part of the body that needed to be operated upon, then bloodletting could not proceed because the Moon might cause excessive bleeding. Some modern surgeons have noted that the Moon does indeed have a small effect on blood flow.
The image above from The Book of Hours (Les Très Riches Heures du Duc de Berry) is early 15th century. This was the most famous of the illuminated manuscripts, which were used as devotionals. From top to bottom you can see Aries, Taurus, Cancer and Gemini, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.