Krishna Dwitiya is the name for the 17th Tithi (lunar day). It's the 2nd Tithi of Krishnapaksha. It's nature is Mangala Prada, which can be interpreted as "one that augurs auspiciousness". It's good for following works: Upanayana, Deva Pratishta (installation of God's/deity's idol or picture), all Paushtik (developmental) and Manglik (the word "Mangala" can be translated from sanskrit as "auspicious") works, Alankara (ornamentation), ornament making and wearing, starting and ending fasts, activity related to house, government related activity, treasury, army, travelling, music, art, sculpture, marriage.
Brahma (a God of creation, first deity of Hindu Triad - Brahma, Vishnu and Shiva) is Devata which rules Dwitiya (according to Muhurta Chintamani and Poorva Kalamrita). Thus, all works that are related to creation or any kind of auspicious undertaking are suitable for this day. As for Brihat Samhita, Krishna Dwitiya is ruled by Vidhata (as per Brahma-Purana - one of Ādityas, son of Daksha and Aditi) or Shrashta (one sacred form of Surya, the Sun).
Being Bhadra (that can be translated as "lucky") and ruled by Buddha (Mercury), the 2nd Tithi of Krishnapaksha 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 months Shravana and Bhadrapada Krishna Dwitiya becomes Maasa Shoonya Tithi. Anuradha Nakshatra also becomes Tithi Shoonya if it falls on Krishna Dwitiya. It's considered that all auspicious works done during Tithi Shoonya periods will be destroyed.
If Surya (Sun) transits through Dhanu (Sagittarius) or Meena (Pisces) during Krishna Dwitiya, it becomes Dagdha (burned) 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.