Krishna Navami is the name for the 24th Tithi (lunar day). It's the 9th Tithi of Krishnapaksha. It's nature is Ugra Prada, which can be interpreted as "increase of aggression". It's good for following works: hunting, poisoning, making of fire arms, fire, gambling; drinking, making alcohol drinks; quarrels, opposition / encounter.
Durga is Devata which rules Navami (referring to Muhurta Chintamani), who's is caring and combining all powers (which are represented as 10 weapons in her arms) for bringing into situations into piece and harmony. As per Brihat Samhita, Krishna Navami is ruled by Nagas (Sarpas) and all actions done in respect of Sarpa deity is acceptable. Regarding Poorva Kalamrita classical text, Krishna Navami is ruled by Ashtavasu.
Being Rikta (that can be translated as "empty hands", "scanty", "destitute") and ruled by Shani (Saturn), 9th Tithi is suitable for following works: arresting, poisoning, uses of weapon, surgery, igniting fire. Travelling in any direction is strictly prohibited and should be avoided during Krishna Navami.
During the vedic month Chaitra Krishna Navami becomes Maasa Shoonya Tithi, which is considered inauspicious in succeeding in works started during this Tithi.
Lagna (ascendant or rising sign) during Krishna Navami becomes Tithi Shoonya (Lagna which is ruined by Tithi) for times when it is Karkata (Cancer) or Simha (Leo). Sometimes (according to Narad Samhita) instead of Karkata Vrshchika (Scorpio) is mentioned. Krttika Nakshatra also becomes Tithi Shoonya if it falls on Krishna Navami. It's considered that all auspicious works done during Tithi Shoonya periods will be destroyed.