More than 3.6 million ballots go to the post office Tuesday in Colorado’s first all-mail election, which includes three national-profile, squeaky-tight races: for governor, U.S. Senate and a U.S. House district in the eastern metro area.

County voting offices have until Friday to get the ballots in the mail, and then voters who plan to mail them back should do so in the weeks before Election Day, Nov. 4. Some key dates:

• Oct. 20 is when voting centers, where voters can drop off ballots, have to be open in each county.

• Oct. 27 is the last day to register and still use a mail ballot.

• Oct. 28 is the last day to ask an elections office for an absentee ballot.

• Oct. 29 is the recommended deadline to put ballots in the mail.

“Voters in the (Denver) metro area can mail as late as Thursday or Friday (before Nov. 4), while rural voters should give themselves a week for mailing time,” said Rich Coolidge, a spokesman for the secretary of state’s office. “When in doubt, deliver your ballot yourself.”

Colorado residents this year can register in person to vote through Election Day. Besides their county offices, residents also can register online at the Secretary of State’s elections site, www.govotecolorado.com, through Oct. 27 in order to get a ballot through the mail.

Residents also can register in person at their county clerk’s office or at a voter’s service center in their county. To register, a person has to be a U.S. citizen who has lived in Colorado for at least 22 days. You need a valid identification, such as a driver’s license, a Colorado ID, military identification card, a U.S. passport, pilot’s license, a student ID with a photo, veteran’s card or tribal ID.

You can also use a copy of a current utility bill, bank statement, government check, paycheck or other government document.

Joey Bunch: 303-954-1174, jbunch@denverpost.com or twitter.com/joeybunch