Krishna Saptami is the name for the 22nd Tithi (lunar day). It's the 7th Tithi of Krishnapaksha. It's nature is Mitra Prada, which can be interpreted as "friend increasing". It's good for following works: marriage; dance, music, singing; jewellery making & wearing; Alankar (ornament and decoration works); hand works; travel; entering house; war; works related to elephants.
Sūrya (Sun) is Devata which rules Saptami (according to Muhurta Chintamani and Poorva Kalamrita). All actions done in respect of Sūrya (health, king, governmental works, helping father etc) are acceptable during this day. According to another classical text, Brihat Samhita, Saptami is ruled by Indra, king of all other Devatas.
Being Bhadra (that can be translated as "lucky") and ruled by Buddha (Mercury), the 7th 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 vedic months Margashirsha and Ashada Krishna Saptami becomes Maasa Shoonya Tithi, which is considered inauspicious in succeeding in works started during this Tithi. Lagna (ascendant or rising sign) during Krishna Saptami becomes Tithi Shoonya (Lagna which is ruined by Tithi) for times when it is Karkata (Cancer) or Dhanu (Sagittarius). Hasta and Moola Nakshatras become Tithi Shoonya if they fall on Krishna Saptami. In case of Nakshatras it's also considered that all auspicious works done during Tithi Shoonya periods will be destroyed.