I have just done it once. In 2020 for the US Presidential election. Because of the pandemic, all voting was to be done by absentee ballot...they mail us a paper ballot and we filled them out and sent them back. Our elections are in November, and we got our ballots, but could not send them in before mid October. Last date to mail them was to be post marked a day or two before election. They could be also dropped in a drop box outside the Board of elections. The only other times absentees are allowed is for medical reasons (like in hospital, or disabled and polling place not accessable. Or if you were going to be out of your area on election day. If memory serves, American astronauts who were "out on a mission" or in Space station could send in ballots prior to departure.