Bhanu Saptami / Surya Saptami is an auspicious day dedicated to Surya Narayan.The Sun God, is an important deity in Hinduism and is worshipped daily and on Shukla Paksha Saptami, falling on a sunday increases the importance.
Bhanu is one of the numerous names of Surya Narayan. It must be noted that Bhanu Saptami is traditionally believed to be the birthday of Surya Narayan. (or the first time Sun God made his appearance). Special prayers and rituals are held on the day in temples. The day is also known as Vivaswat Saptami also. Vaivaswat’s descendants are called as Suryavanshis, because they are originated from the Sun God.
Hence, Vivaswat Saptami is also observed as Bhanu Saptami. Vaivaswata Manu was the descendant of Vaivasvan, who ruled Ayodhya. Vaivasvans descendants made their rule on India. Descendants of Vaivasvans sons were called Suryavanshis. Shri Ram being Suryavanshi.
Vaivaswatha Manus daughter, Ella, was married Buddha, the son of Chandra. Her descendants were called Chandravanshis. Shri Krishna being Chandravanshi. Bhanu Saptami / Surya Saptami / Ravivaari Saptami vishesh yog hai.