Early Voting

I apologize if we covered this earlier in the semester and I forgot, but..
On page 172 of Defying the Odds, there is a discussion about early voting in the 2016 election. An estimated 40% of voters cast their ballots before election day, before major turning points in the race occurred. How early are people allowed to start voting this way? Can the date that people are allowed to start early voting be changed, so that people cannot vote until maybe just a week or two before?