The law takes effect (or simply 'effective' i.e. "effective immediately" or on a certain date)... The law is implemented/enforced/enacted. Although in my opinion "implemented" is best suited here than the other two variants, since it is more neutral. "Enforced" or "enacted" implies application of the law in reaction to an action (crime/infringement followed by punishment), whereas "implemented" does not carry such connotation... It is in essence just a message of a legal regulation