One website I have been following for a year or so is Free Preview TV. You can sign up to get notifications of free previews of TV channels on DirecTV, DISH Network and others
This weekend DirecTV is having a free preview weekend of HBO, Cinemax, Starz, Showtime and Sports Pack. One of the comments left on the post states that DISH Network is having the preview also. On DirecTV the channels are 500-657. The preview runs from Friday, December 16 to Monday, December 19.
You can signup to get emails about upcoming previews.