According to the Law Offers wiki only two people on the forum have got interviews for Law so far and nobody has had an offer/rejection. It could be that your invitation to interview is coming in the next few days, or maybe they're waiting to see what other applicants they get before deciding whether to give you an interview or even an offer without interview.
It's perfectly normal to wait this long, it's only been a month! Some people apply in October and don't hear from a choice until April.