In the most important day story from The Miracle Worker, how was the mystery of language revealed to Helen Keller?
Although there is no section labeled clearly as "the most important day story," I will assume that you mean the last day when Helen truly does discover the mystery of language through the word "water." With that being said, here is my answer:
Although one cannot deny the teachings of Annie Sullivan, the most wonderful revelation of The Miracle Worker is that the mystery of language wasn't revealed to Helen Keller, she discovers it's beautiful mystery on her own.
The way this happens is through yet another feud and fight between Helen and her teacher. Both are frustrated. Both are at their wits-end. Helen has spilled a pitcher of water all over Annie, and Annie insists on Helen filling it again at the outdoor water pump. Annie haphazardly continues signing water under Helen's hand as she fills the pitcher. Suddenly Helen stops and understands that the wet liquid pouring over her hands is, in fact, this sign for "w-a-t-e-r." Helen is still for a long time before attempting to say what was originally her first word: "Wah. Wah." Suddenly, she begins frantically signing the word again and again to check its truth. Once her mind is opened, Helen cavorts about the yard asking for names of other things: truly a miraculous scene.
Ironically, the traditional opinion would be to say that Annie Sullivan was the one who revealed the mystery of language to Helen. What I suggest is that, although Annie didn't reveal the mystery to Helen, she certainly handed her the key to that mystery. This is nicely complemented by the touching scene when Helen hands the real keys to Annie.