Importance of Ananta Vrata as explained by Shri Guru Narasimha Saraswati himself to one of his Close Disciple Sri Sayamdeo (Ref. Adhyay 43 of Shri Guru Charitra)
Ananta Vrata is celebrated on the day of Bhadrapada Shuddha Chaturdashi every year. The Significance of the vrata was explained by Shri Guru to Sayamdeo and his Family. Shri Guru begins by saying him how Lord Shri Krishna himself advised the Pandavas to perform the Ananta Vrata during their Vanavas to get back the Kingdom of Hastinapur. Shri Guru also quotes the Story which was narrated by Shri Krishna :-
Shri Krishna says, Once in the Krita Yuga there lived a Brahmin named Sumantu who belonged to Vashishta Gotra and married to the daughter of Bhrigus Maharishi called Diksha. They had a daughter called Susheela. After some years his wife Diksha died and all the Nitya Karma’s of Sumantu stopped because of it and he had to marry again with another women called Karkasha. She always quarreled with them and Sumantu was upset due to this. Once Koundinya Rishi arrived there on his way and Sumantu married his daughter Susheela with him. Koundinya Rishi stayed there for two months and soon realizing the Conflicts between his wife and Mother in Law, he left their home.
On the way the Sage stopped nearby a River for Anushthan and Susheela joined the other womens there and performed the Holy Ananta Vrata along with them till her husband returned.
Lord Ananta was pleased and due to his grace the Rishi became the Head of the nearby Amaravati village. Once when both of them were sitting together Rishi Koundinya noticed a red thread on the wrist of his wife Susheela and blamed her thinking she is trying to control him using it. Though Susheela tried explaining the Fact, still Koundinya Rishi threw it in the fire angrily saying their prosperity is all because of his Penance and not because of Ananta Vruta. Due to such action the Rishi lost all his wealth and soo realized his Mistake. He left for the forest searching Ananta. On the way he met many creatures, trees, rivers etc who were all punished to become so because of some mistakes committed in their previous births. In despair the rishi tried to kill himself but Lord Ananta appeared in the guise of an old man and Anant took him to a Fine City made of Gold and showed his real form and blessed him with Prosperity again.
Thus the story was narrated by Shri Krishna to the Pandavas and by Shri Guru to his disciple Sayamdeo. But Sayamdeo says with firm conviction that “Shri Guru himself is Ananta then why perform any vruta / puja instead of serving him” but still Shri Guru insists him saying “you must perform the vruta as your Gotra too is Koundinya”. As per the words of Maharaj Sri Sayamdeo performs the Vrata and also a Grand Samaradhana at Ganagapur. It can be noted here that the Ananta Vrutam is performed by some priest families even till today at Sri Kshetra Ganagapur.