Is there always a temperature "dip" on the day of ovulation?

No. Many people do see a dip the day of ovulation, but many people do not. A temperature dip cannot confirm ovulation. More important than observing a dip, is seeing a temperature rise after ovulation. Your temperature rises in response to the presence of the heat-inducing hormone progesterone, which is only released after ovulation. If you are lucky enough to have advanced warning of ovulation in the form of a temperature dip, take advantage and be sure to have intercourse that day.