What, according to Helen Keller, was the most important day of her life?

In the first lines of chapter 4, Helen Keller states,

The most important day I remember in all my life is the one on which my teacher, Anne Mansfield Sullivan, came to me . . . It was the third of March, 1887, three months before I was seven years old.

Up to that day, Helen's communication was severely limited. She did not know the meaning of any words and mostly experienced life through various sensations. For example, she knew who was walking toward her based on feeling vibrations. At the beginning of chapter 4, she can vaguely guess something important is going to happen because her mother is "hurrying to and fro in the house."
It is telling that when Ann Sullivan arrives and leads Helen into the house, she mistakes her for her mother. This suggests she feels comfortable and at home with Ann from the very beginning.
Ann goes on to teach Helen the words for "doll," "water," and "mug" by tracing the letters of the words on Helen's hands. Far from being what Helen calls an angry and bitter child, she suddenly becomes "joyous and confident."

I recall many incidents of the summer of 1887 that followed my soul's sudden awakening. I did nothing but explore with my hands and learn the name of every object that I touched; and the more I handled things and learned their names and uses, the more joyous and confident grew my sense of kinship with the rest of the world.

You can find the answer to this at the beginning of chapter four. Helen says that the most important day of her life, as far as she can remember, was the day Miss Anne Sullivan arrived to be her teacher. This made an enormous difference in Helen's life, and without Anne Sullivan's teachings, it is unlikely that Helen would ever have been able to write her book at all. Helen describes her existence before Miss Sullivan's arrival as being comparable to being at sea in a fog: she was aware of elements of life, such as the sun on her face, but she felt generally lost, anxious, and unsure of what was going on around her or what she could do about it. Anne Sullivan was the person who put Helen in touch with the rest of the world, teaching her to communicate, read, and understand her place in her world.