Anything that's rabbinical has to to with Jewish law or with rabbis. If you go to a religious school and plan to become a rabbi, you are getting a rabbinical education.
You can more or less use the adjectives rabbinical and rabbinic interchangeably. A rabbinical scholar is learning how to be a rabbi, or a Jewish religious leader; and the Hebrew or Jewish calendar can also be called the rabbinical calendar. In Mishnaic Hebrew, rabbi, the root of rabbinical, means "my master," from the Aramic rabh, "master" or "teacher."