This depends completely on where you are.
If you are in the path of totality, then you first see the beginning of the partial phase. (This is called “C1” by astronomers) The Moon’s “bite” out of the Sun will grow slowly, until the moment you’ve been waiting for: totality!
The beginning of totality is called “C2” by astronomers.
If you are NOT in the path of totality, then you will see C1, but that bite will grow until it gets a maximum size, then it will shrink again and the eclipse will be over. It will never be total.
But you asked “when” it would happen! Since you now know that you must be in the path to see totality, and that all these events happen at different times in different locations, you are ready to go to the best places there are for finding out WHEN:
Check out the eclipse2017.org Android app – which gives you all you need to know!
Check out our Instructions page – how to watch the eclipse!
Check on www.eclipse2017.org for the actual times of totality from your location.