There are three key categories of voting in the United States. 

In most states, citizens sign up as voters prior to Election Day, with a cutoff for registration somewhere between 3 and 30 days before the election, but the policy  varies from state-to-state regarding requirements, sub-categories and deadlines:

 

Voter Registration : Registration by mail, online or same-day registration.

 

Early Voting:  Process for voting in advance of the scheduled election day - by mail, online or in person at a designated polling location.

 

Absentee Voting:   Where an individual who expects to be absent on election day may vote by mail, which entails an absentee ballot request deadline, as well as ballot submission deadline.

 

All 50 states and the territories, including Democrats voting abroad, are still open to voter registration. For your convenience, the deadlines for each!