This interesting article in The Indian Panorama discusses how the Sun God evolved and his possible link to Central Asia. You can find the entire piece here.
In Roman times, Sunday was the first day of the week —an important day named after the Sun-God. Helios or Hyperion was the Greco-Roman Sun-God, who later merged with another Greek god, Apollo, the youthful, energetic, beautiful god who shot arrows that got rid of the darkness. When the Roman empire became Christian, the day of the Sun became Dominica, the Day of the Lord, the day when — according to Christians — God rested after creating the world in six days.
Unlike the months of the year, weekdays have no astronomical correlations. It is an arbitrary division of time, believed to have its origin in Babylon from where it spread eastwards to India and thence to China and westwards through Rome to around the Mediterranean. Just as Romans attributed the first day of the week to the Sun, so did Indians who called it Ravi-vaar. Why was the first day of the week associated with the Sun, no one knows. It is one of those mysteries of history that remain unresolved. The earliest reference to Sunday in India comes from texts dated after 400 CE, and it is believed to have come with Sun-worshippers like the Huns, Parthians and Scythians, who entered India around the time of the Gupta kings.