Setting is just another word for putting eggs into an incubator or under a broody hen.  Traditionally chicks which hatch under a waxing moon – that is when the moon is growing – will be bigger and healthier than those hatching when the moon is waning or growing smaller. So, according to some old-time farmers, fertile eggs should be set 21 days before there's a new moon. Of course, there's no scientific basis for this ...