Many thanks to Eileen Naumann for her book Medical Astrology, from which the following is quoted. (Footnote is mine)

The positions of the planets and luminaries often determine the best and worst times for a person to undergo surgery. Following are guidelines for planning operations astrologically:

  1. Try to plan an operation five days before or after the new Moon. At this time, fluids are at their lowest ebb; consequently, there is less chance of swelling.
  2. Avoid operations five days before or after a full Moon. At this time bodily fluids are at their highest and can cause excessive swelling, hemorrhaging or seepage from wounds.
  3. A day in which the Moon is void of course is a bad one for surgery. On such a day there is a good possibility that the operation won’t be performed correctly, that complications will arise or that a second operation will become necessary.
  4. Avoid an operation on that part of the body ruled by the sign in which the Moon is transiting1.
  5. Avoid surgery when the Moon is in a mutable sign— Virgo, Gemini, Pisces or Sagittarius.
  6. Try to plan an operation when the transiting Moon is in a fixed sign— Taurus, Leo, Scorpio or Aquarius. With such a placement the operation should go as planned, the surgeon’s hand should be steady, and no further complications should arise.
  7. Avoid surgery when the transiting Moon is combust or within 17 degrees of the natal Sun, Moon or Mars.
  8. Avoid surgery when the transiting Moon is square, opposite or inconjunct the natal or transiting Sun, Mars, Saturn, Neptune, Uranus or Pluto. Mars in a tension aspect with the transiting Moon can mean excessive bleeding or inflammation after surgery. Saturn can mean chronic or very serious complications.
  9. Try to plan an operation when the transiting Moon is sextile or trine to natal, progressed or transiting Venus, Mars, or Jupiter. Such an aspect will help the surgery go smoothly. The transiting Moon sextile or trine Mars will also help insure that the surgeon will have a quick, clean cutting hand and will know what he or she is doing.
  10. Avoid surgery when Mercury or Mars is retrograde. Mercury retrograde can mean misunderstandings, mistakes, and confusion. Since Mars is the planet of surgeons, when it goes retrograde surgeons tend not to be at their steadiest or most reliable: they may not be able to concentrate well. Furthermore, Mars retrograde can mean heavy loss of blood.

1 Every sign corresponds to a part of the body. For example Aries is associated with the head. Therefore, Moon in Aries is not the time to operate on the ears, jaw, brain, eyes, head, or face.