What is nidation? In which phase of the menstrual cycle does nidation occur?

Nidation is the implantantion of the embryo in the uterus. Nidation happens around the 7th day after fecundation, i.e., 7 to 8 days after ovulation (obviously, it happens only if fecundation also occurs). As it occurs in the luteal phase the progesterone level is high and the endometrium is in its best condition to receive the embryo.