Every year on Shukla Sapthami in magha masa the Dodda Rathothsava (big chariot festival) of Shri Ranganatha Swamy will be celebrated. The festival commences with Ankurarpane, hoisting of Rathosava flag followed by different Uthsava in the night inside the premises of Shri Ranganatha Swamy Temple. On the 5th auspicious day (Thirunal) reading of Prahladotsava Purana (story) are conducted. On the 6th auspicious Day (Thirunal) Gajendra Moksha Utsava will be performed. On 7th day morning at 6 6:00 AM Soorya Mandal Vahanotsava will be performed. Gajalakshmi utsava will be performed to Goddess Shridevi taking in a procession along the main roads of the town. After that Mantopothsava will be performed to Lord and Shridevi.
Around 1:00 PM after special pooja the lord will be seated in the chariot Rathothsava will be performed pulling the chariot one round in Ratha beedhi around the temple. On this occasion thousands of devotees from different parts assemble in the temple to witness the Rathothsava and it is a followed custom to throw fruits/ Jawana (a kind of fragrant plant). On the ninth day (thirunal) a Avabrutha Snana (bath) seva will be performed followed by descending of flag which ends the Brahma Rathothsava.
Saptami Tithi is dedicated to Lord Surya. Shukla Paksha Saptami in Magha month is known as Ratha Saptami or Magha Saptami. It is believed that Lord Surya Dev started enlightening the whole world on Ratha Saptami day which was considered as birth day of God Surya. Hence this day is also known as Surya Jayanti.
Ratha Saptami is highly auspicious day and it is considered as auspicious as Surya Grahan for Dan-Punya activities. By worshipping Lord Surya and observing fast on this day one can get rid of all type of sins. It is believed that seven types of sins done, knowingly, unknowingly, by words, by body, by mind, in current birth and in previous births are purged by worshipping Lord Surya on this day.
On Ratha Saptami one should take bath during Arunodaya. Ratha Saptami Snan is one of the important rituals and is suggested during Arunodaya only. Arunodaya period prevails for four Ghatis (approx. one and half hour for Indian locations if we consider one Ghati duration as 24 minutes) before sunrise. Taking bath before sunrise during Arunodaya keeps one healthy and free from all types of ailments and diseases. Because of this belief Ratha Saptami is also known as Arogya Saptami. Taking bath in water body like river, canal is preferred over taking bath at home.
After taking bath one should worship Lord Surya during sunrise by offering Arghyadan (अर्घ्यदान) to Him. Arghyadan is performed by slowly offering water to Lord Surya from small Kalash with folded hand in Namaskar Mudra while facing Lord Sun in standing position. After this one should light Deepak of pure Ghee and worship Sun God with Kapoor, Dhup, and red flowers. By doing morning Snan, Dan-Punya and Arghyadan to Suryadev one is bestowed with long life, good health and prosperity.
This day is also known as Achala Saptami.