Shukla Dwadashi is the name for the 12th Tithi (lunar day). It's the 12th Tithi of Shuklapaksha. Its nature is Yasha Prada, which can be interpreted as "fame giver". It's good for following works: movable or fixed works; peaceful or Paushtik works, first feeding, Upanayana, marriage. Travelling, entering into new house and oil massage should be avoided.
Vishnu is Devata which rules Dwadashi (according to Muhurta Chintamani and Poorva Kalamrita). Vishnu is Devata who stays for preservation, he's a divine power keeper. As per Brihat Samhita, Shukla Dwadashi is ruled by Savitar (one form of Surya, the Sun) and all actions done in respect of Savitar or Surya (health, king, governmental works, helping father etc) are acceptable during this day.
Being Bhadra (that can be translated as "lucky") and ruled by Buddha (Mercury), the 12th Tithi of Shuklapaksha is suitable for following works or actions: marriage, Upanayana, ornament making and using, clothes; art; travelling by elephant or horses, travel, conveyance; making of roads; oil massage.
During the vedic month Magha Shukla Dwadashi becomes Sompad (that can be translated as wealth, possession, success) Tithi, which is considered very auspicious in succeeding in works started during this Tithi. Also in vedic month Kartika Shukla Dwadashi is called Tamas - Manvadi Tithi, which is considered to be extremely auspicious for any Punya Karma (Japa, Tapa, Dana, Havan, Homa etc.). But being Manvadi Tithi Shukla Dwadashi should be avoided in starting of education, Upanayana, marriage, ending of fasts, construction of house and entering thereof; travelling; plantation works.
During the vedic month Vaishakha Shukla Dwadashi becomes Maasa Shunya Tithi, which is considered inauspicious in succeeding in works started during this Tithi.
Ashlesha Nakshatra becomes Tithi Shoonya if it falls on Shukla Dwadashi. It's considered that all auspicious works done during Tithi Shoonya periods will be destroyed.
If Surya (Sun) transits through Tula (Libra) or Kumbha (Aquarius) during Shukla Dwadashi, it becomes Dagdha Tithi. Dagdha Tithis are known as inauspicious for starting any kind of work. Rishi (sage) Brihaspati also says that if Dagdha is present either for Lagna or Tithi then it inflicts death.