The 12th house represents your subconscious and hidden behaviors (sometimes called your undoing). It is also a house of endings. Saturn in Sagittarius transiting this house will likely coincided with the gradual end of something in your life, although it may be hard to predict because this house is your blind spot. There could be victim or martyr behavior you must give up, or something that needs to be released by way of a personal sacrifice.
If you have Saturn in your 12th house currently, you likely have Scorpio or Sagittarius on the cusp. Scorpio-cuspers are extra private, and may not enjoy looking at their own dark side. You access your subconscious via psychology, astrology or occult means. Saturn’s presence may bring especially painful experiences to you, because you want full control of your shadow side, which ironically slips through your fingers. You may not consciously realize what is ending in your life, except in dreams, but can get a clearer understanding via a tarot or oracle card reading, or talking with a psychic or medium. This will make you feel more powerful.
Sagittarius-cuspers are more light-hearted and carefree about unconscious behaviors. You can benefit from a walking-meditation or yoga; anything physical that lets your mind become free to wander. You may even be great at astral travel. Saturn’s presence can reveal a major blind spot, however, that you will never see coming due to your faith. It may stem from over-enthusiasm, or being too preachy about your beliefs; someone may not be on your level and walk out of your life, and it completely catches you off guard. Your biggest lesson may be that some people don’t want to hear your “truth” because it isn’t their own.
For either of you, this transit can be a confusing time, because the 12th house is hard to grasp. New relationships are not likely to blossom during this time; you are in fact cutting off excess in your life and may end superfluous relationships, or ones that aren’t in alignment with your beliefs. Relocation to a far away place is quite possible, as are changes in study, religion, philosophy, and spiritual beliefs.