I may be wrong but I don't think the Orthodox Church has any “laws” concerning the Fast of Great Lent, it's really a Tradition. The fast is an opportunity to renew spiritual committment and prepare for the Feast of Great and Holy Pascha.

During Great Lent, Orthodox Christians are encouraged to abstain from eating all meat and dairy products each and every day. Lenten meals are also traditionally smaller meals. Fortunately, the cultures in which these Traditions originated invented a number of wonderful Lenten dishes to take up the slack.

Since the Roman Catholic and Orthodox churches were one until 1054, I assume Roman Catholic tradition followed Orthodox practice for the first thousand years. It may be, though, that Western practice was already diverging prior to that time.

The Orthodox Church observes many more fasts than just Great Lent. In fact more than half of the days in each year are fast days.

      - Mark