In the federal system that is the United States, each individual state determines voting processes which is why so many Republican-controlled states have passed the voter suppression laws.
So each state determines whether a vote can be cast before polling day either through the post or otherwise. Votes are not actually counted though until the close of polls on polling day.
I understand that nation-wide more than 5 million votes have already been cast including a third of the eligible votes in the all-important state of Ohio. Obama himself has already voted in Chicago.
Just to complicate things still further, the presidential election is not determined by the number of voters who support each candidate but by the votes in the Electoral College which technically does not meet until December and technically does not have its votes validated by Congress until early January.
As you know, this is the greatest democracy on earth …