Why does Helen refer to the day as an eventful one in The Story of My Life?

A few synonyms for eventful in this context would be: significant, momentous, important, historic, consequential, and fateful. All these words imply that the day which Helen refers to stood out from anything else that happened before and that it would influence her destiny to such an extent that her life would be irrevocably changed forever from that day. The day would forever stand out as something special. 

At the beginning of chapter four, Helen mentions...

A few synonyms for eventful in this context would be: significant, momentous, important, historic, consequential, and fateful. All these words imply that the day which Helen refers to stood out from anything else that happened before and that it would influence her destiny to such an extent that her life would be irrevocably changed forever from that day. The day would forever stand out as something special. 

At the beginning of chapter four, Helen mentions that the day, 3 March 1887, was to be the most important in her life, which emphasizes the point made above. She specifically states the following:

The most important day I remember in all my life is the one on which my teacher, Anne Mansfield Sullivan, came to me.

The arrival of Ms. Sullivan would have a dramatic impact on Helen's life. Helen herself declares in the text that Ms. Sullivan "was to set my spirit free." She had been selected to teach Helen on the advice of Mr. Anagnos after Helen's father had asked him whether a teacher for Helen, who was both deaf and blind, could be found. Ms. Sullivan had, at the time, been in the service of the Perkins Institute in Boston, where Dr. Howe, "who had discovered the way to teach the deaf and blind" had achieved amazing success. He had left behind a legacy of methods to help those devoid of sight or hearing. 

Ms. Sullivan's teaching would, through her hard work and dedication as well as Helen's desire to learn, result in Helen's discovering how to read, write, and speak, despite the major disabilities that she had. Helen would become an inspirational public speaker and an outstanding example of someone who has beaten the odds. In the process, she became a guiding light, a catalyst, and a muse for many disenchanted and disabled individuals throughout the world, not only during her lifetime but, most certainly, for all time.