Krishna Ashtami is the name for the 23th Tithi (lunar day). It's nature is Dawanda Prada, which can be interpreted as "conflict or problem generator". It's good for works related to women, Alankar, enjoyment. It's auspicious for dancing, making and wearing jewellery, art, sculpture, painting, writing, house, foundation laying, war, government works and celebrations, trade, farming, stone, steel, water related works. Eating of meat should be avoided during this Tithi.
Shiva is Devata which rules the 8th Tithi of Krishnapaksha (referring to Muhurta Chintamani and Poorva Kalamrita), which is good for the destruction of any obstacle or enemy, overcoming of any problem and acts of combat. As per Brihat Samhita, Krishna Ashtami is ruled by Vasu and all actions done in respect of Vasu are more than acceptable.
Being Jaya (that can be translated as "victory" or "advancement") and ruled by Mangala (Mars), 8th Tithi of Krishnapaksha is suitable for following works or actions: weapon making; wars; training in army, collection of army, any matters related to power or strength; travelling; festival; medicine; trades; house matters; flags & conch-shell.
During the vedic month Shravana Krishna Ashtami becomes Indra Savarni - Manvadi Tithi, which is considered to be extremely auspicious for any Punya Karma (Japa, Tapa, Dana, Havan, Homa etc.). But it should be avoided in starting of education, Upanayana, marriage, ending of fasts, construction of house and entering thereof; travelling; plantation works.
Being the 8th Tithi of Krishnapaksha Krishna Ashtami is also called Parva Tithi which can be translated as "festival lunar day". It's considered that Parva Tithis should be avoided in all auspiscious elections.
During the vedic months Chaitra and Margashirsha Krishna Ashtami becomes Maasa Shoonya Tithi, which is considered inauspicious in succeeding in works started during this Tithi. Poorva Bhadrapada Nakshatra also becomes Tithi Shoonya if it falls on Krishna Ashtami. Krishna Ashtami also becomes Daghda Tithi (burned lunar day) for times when Surya (Sun) goes through Mithuna (Gemini) or Kanya (Virgo). It's considered that Daghda Tithis are inauspicious to start any type of work.