Rath Saptami is dedicated to Lord Surya or Hindu Sun God and is observed on the Magh Shukla Saptami (seventh day during Waxing phase of moon in Magh month). The popular belief is that Lord Surya rode a chariot tied with seven horses to annihilate demons Mandeha. This is the popular story associated with Ratha Saptami and this Katha is listened to by people on the day.
Some stories suggest that Mandeha a group of demons tried to overpower Surya. But he was able to kill them with his light. It is believed that Surya rode the chariot and annihilated the demons on Magh Shukla Saptami day.
(Please note that there are numerous version to the above story and one story suggest that the demons try to block Surya daily but he fights them and moves forward with the strength of ‘argya’ performed early morning by humans on earth.)
Another popular story indicates that Vishwakarma – the divine architect – had built the Chariot of Surya on the day. This story suggests that the chariot was built for Lord Shiva to fight demon Tripurasura and it was later gifted to Lord Surya.
The chariot of Lord Surya has seven horses and the Sarathi (chariot driver) is Aruna. Some scholars indicate that the Chariot has only one wheel.
You can read more about the importance of Ratha Saptami in this article.