When you have a natal planet in the 12th house, the energy of that planet tends to get gobbled up and doesn't show through in your outward personality. People with the Sun, Moon, Mercury, and women with Venus or men with Mars in the 12th house tend to come across as very shy people, even if their planets are in fire or air signs (outgoing/sociable). In those cases, it tends to be that the person comes across as very shy, quiet, introverted, but when alone with people they're comfortable with and in environments they're comfortable with, then you get to see how rambunctious they are. 12th house people tend to do well behind the scenes, and shy away from the spotlight.
In prediction, when a transit (moving) planet is going through the 12th house, then the energy of that planet is temporarily hidden. If the Sun is going through your 12th house, you may find yourself spending more time alone, or feeling more shy; if Mercury going through your 12th house, you tend to keep quiet about things more, not wanting to voice your opinion, and engage in secret-keeping. The slower moving planets tend to show a letting go, as this house rules endings. You may find yourself getting rid of a lot of "baggage."
January 2019 New & Full Moons
Jupiter in Sagittarius: November 2018 - December 2019
Saturn in Capricorn: December 2017 - March 2020, July - December 2020
Uranus in Taurus: May - November 2018, March 2019 - July 2025, November 2025 - April 2026