What was the most important day in Hellen Keller's life? How did she learn to communicate for first time since becoming blind and deaf?

The most important day in Helen Keller's life was when Miss Sullivan's attempts to communicate with her were finally successful. Miss Sullivan repeatedly tried fingerspelling into Helen's palm to help her associate words with objects. Helen was confused and thought of it as more of a game. One day, Miss Sullivan took Helen to an outdoor water pump on the Keller property. She held the girl's hand under it and spelled "w-a-t-e-r" onto her palm as the liquid poured out. For the first time, Helen made the connection between what was spelled into her hand and the water that was flowing through her fingers. Helen described that moment in her autobiography The Story of My Life:

I felt a misty consciousness as of something forgotten — a thrill of returning thought; and somehow the mystery of language was revealed to me (Chapter Four).

After that moment, Helen became eager to learn the names of everything around her. This was when Helen became a life-long learner.