The PSA: Get an Absentee Ballot, If You Will Need One
New Yorkers: today is the last day you can mail off for an absentee ballot — download the form at the NYC Board of Elections site and have it postmarked today, or walk into one of their offices (the Brooklyn one is around Borough Hall) before 5pm. (You can also vote early, on weekdays, at said locations, if you're registered here.)
If you're living here but from somewhere else in the state, here is the state Board of Elections page for it, to mail today to your County Board.
Out-of-staters check your own state's BoE site for absentee info, but today would seem to be the day.