Shani Sewa Samiti 3.17

The word shani also denotes the seventh day or Saturday in most Indian languages. The word shani (शनि) comes from Śanayē Kramati Saḥ (शनये क्रमति सः), the one who moves slowly, because Saturn takes about 30 years to revolve around the Sun.
Shani is a deva and son of Surya and his wife Chhaya, hence also known as Chayyaputra. He is the elder brother of Yama, the Hindu god of death, who in some scriptures corresponds to the deliverance of justice. Surya's two sons Shani and Yama judge. Shani gives us the results of one's deeds through one's life through appropriate punishments and rewards; Yama grants the results of one's deeds after death.
It is said that when Shani opened his eyes as a baby for the very first time, the sun went into an eclipse, which clearly denotes the impact of Shani on astrological charts. He is known as the greatest teacher and well wisher for the righteous as well the greatest punisher for those who follow the path of evil, betrayal, backstabbing and unjust revenge. He is depicted dark in colour, clothed in black; holding a sword, arrows and two daggers and mounted on a crow, which is Shani's vāhana. As protector of property, Shani is able to repress the thieving tendencies of birds.