Not all of the Moon's effects on health are considered negative. Some suspect that the flow of blood responds to the gravitational tug of the Moon.
Believers in the power of the Moon consider dental care and elective surgery unwise around the times of the full Moon because of the risk of increased bleeding. (Civil War doctors noted greater blood flow during a flood tide.)
Do you get the blues on Mondays? Maybe you're engaging in the wrong activities. Literally, the “Moon's day,” Monday in legend and lore is auspicious for peace and happiness, raising spirits of the dead, and other activities. So if it's Monday, shoot for the Moon!