Why was the summer of 1817 special for Helen Keller?

I believe you're referring to the summer of 1887, as this was indeed a very special time for Helen Keller. This was the moment when Annie Sullivan helped her to achieve a breakthrough in communication. Prior to Annie's entering her life, Helen had limited ability to communicate and make sense of the world around her. But Annie gradually introduced Helen to a whole new language, one where simple words could be spelled out on the palms of her hands.
Nevertheless, progress was inevitably slow and Helen's relationship to the outside world remained limited. That all changed one summer's day in 1887. Helen is out walking with Annie and they come to a well-house. Annie places Helen's hand underneath a water spout. As Helen feels the cool, refreshing gush against her hand, Annie takes her other hand and spells out the word "water" on her palm. All of a sudden, what had previously been the mystery of language has now been revealed to her. Helen can now associate that cool, running liquid with the word "water." That simple, unforgettable experience gives Helen enormous hope and energy, establishing a connection between herself and a world previously inaccessible to her. It also kindles a great passion for learning, one that would form the basis of her many educational achievements to come.