If you're new to Groove Music, Microsoft wants to tempt you into the fold with 4 months of Groove Music Pass for free. That'll net you unlimited streaming of millions of songs, playlists, radio and more for 4 months without paying a dime.
All you have to do to get started is to sign up for a Groove Music pass. After the initial 30-day free trial is up, Microsoft will then send you a promo code to redeem an additional 3 months of Groove Music pass. Keep in mind, however, that you will still have to provide credit card information on sign-up so the subscription can continue once the trial period is over. Of course, you can always cancel service before the 4 months is over to avoid being charged.
Do note that this is for new subscribers only; if you've previously subscribed to Zune, Xbox or Groove Music, you're unfortunately left out of this deal. If you're interested, however, you can sign up and grab your free 4 months of Groove Music Pass at Microsoft through December 31.