The "You" represents God, to whom the poem is addressed.
This poem is a prayer of thanksgiving, spoken directly to God, thanking Him for the beauty of the natural world, "for everything which is natural which is infinite". The second and last stanzas also communicate a sense of joy in rebirth, for which the speaker is also thankful. The speaker, who has metaphorically "died" is "alive again today", and everything in the world seems fresh and new to him, his "ears...awake and the eyes of (his) eyes opened". In his inimitable way, Cummings in his exuberance questions how individuals, even though "merely" human, can doubt the existence of God, again addressed as the "unimaginable You". The "You" in the poem is clearly a God of amazing power, joy, and love. The fact that the word is capitalized both times when it appears in the poem reinforces the idea that it is being used as a name, and also may indicate the high esteem with which the poet holds for the deity.
The poem also juxtaposes the positive and the negative in beautifully creative and metaphorical imagery. All things which are good and beautiful - the "amazing day", the trees, the sky, the sun, and life and love and "gay great happenings" on earth - are affirmed with a simple "yes" which is "infinite", while all that is negative is "the no of all nothing". Because of the power of God, the "unimaginable You", human beings have been "lifted from the no of all nothing" to celebrate a life in a world which is a definite "yes".