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First, always consult your

First, always consult your physician when planning surgery.
Capricorn influences the knees.  You want to avoid dates around the time when the Moon is in Capricorn. See the annual Old Farmer's Almanac for the Moon's Sign Calendar:
http://www.almanac.com/store/almanacs

For instant access to the Moon's signs download our 15-month Moon's Sign Calendar here for a small fee: www.almanac.com/product/2014-astrology-moons-sign-calendar