Ratha Saptami is also famous as- Achala Saptami, Surya Jayanti and Magha Saptami. As per Hindu narrations, Ratha Saptami Vrat Katha is basically a mythological tale of a lady-
There is a legend in the Bhavishya Purana that InduMati, a courtesan, had never done any charity in his life. When it came to its end time, Vashisht added to the sage and asked for ways to get rid of Vashistha Muni, then the muni said to the courtesans in the north- do the fast of Achala Saptami (Achala Saptami fast) on the seventh day of Magh month. For this, eat only once on the day of Shashthi and before the bath on the morning of Saptami, place seven leaves of Aak on the head and meditate on the sun and shake the water from the sugarcane, Rup Namaste Rudraupaya Rasanam Patay Namah] Varunaya Namah Stu After uttering the mantra, pour the lamp into the water.
After bathing, make an Ashtadali statue of the Sun and install Shiva and Parvati and worship them methodically and donate them by filling rice in a copper vessel. Ganika Indumati observed Magh Saptami fast according to the statement of Muni. After sacrificing her body by virtue, Indra made her the heroine of the Apsaras. Those who cannot bathe in the river can bathe by putting Ganga water in the water.
According to another legend, Lord Shree Krishna's son Shamb was proud of his strength and physical beauty. Once Durvasa Rishi came to meet Shri Krishna, at that time the body of Durvasa Rishi had become very lean because he was absorbed in difficult tenacity for a long time. Shamb laughed seeing Rishi. By nature, Sitali got angry at Shambha's profanity and cursed Shamb for being leprosy. The curse of the sage took effect immediately. When there was no benefit from the treatment, then Krishna advised Shamb to perform Suryopasana, soon after Suryopasana, Shamb got rid of leprosy.