The initial deadline, Dec. 15, was extended until Dec. 23, and then extended another 24 hours Monday in response to heavy traffic at the website.
Many of the 14 states operating their own healthcare exchanges complied and extended their deadlines 24 hours to 11:59 p.m. Tuesday after Monday's extension was announced.
"Anticipating high demand, which we are currently experiencing, and the fact that consumers may be enrolling from multiple time zones, we have taken steps to make sure that those who tried to enroll [Monday] but had delays due to high traffic have a fail-safe," said Julie Bataille, spokeswoman for the U.S. Centers for Medicare and Medicaid Services, the federal agency overseeing the national health insurance exchange.
Some officials compared the last-minute 24-hour deadline extension to keeping election polling places open extra hours to make sure people already in line at closing could still vote, the New York Times said.
The Washington Post and the Wall Street Journal said HealthCare.gov was changed to let users sign up for the first wave of coverage over the weekend.
Insurers said they received no warning about the deadline change and hadn't prepared for it. The change was not announced by CMS until the Post reported the extension Monday.
The flood of online visitors triggered a backup queuing system that invited people to come back during less busy times. More than 60,000 people left email addresses so they could be alerted when the site wasn't so busy, Bataille said.
Even with the record-setting number of users, the troubled and much-maligned health portal handling insurance sales for 36 states managed the traffic well and reported few errors, CMS said.
And despite the last-minute deadline adjustment, Tuesday's federal website deadline for coverage starting Jan. 1 is final, administration officials said.
The same applies to most states operating their own marketplaces, although two states have later deadlines.
Oregon has delayed its sign-up deadline for Jan. 1 coverage to Dec. 27. Minnesota is giving consumers until New Year's Eve to choose coverage starting the next day.
Still, this month's deadline is not the last chance to get insurance through the marketplace, officials stressed.
The open-enrollment period that started Oct. 1 runs through March 31. The difference is, coverage booked through HealthCare.gov after 11:59 p.m. Tuesday starts a month later.
President Barack Obama said Friday more than 500,000 people signed up at HealthCare.gov in the first three weeks of December.
The figure was greater than the total number of enrollments for October and November combined, and brought the number of people enrolled in the Patient Protection and Affordable Care Act, commonly known as "Obamacare," to more than 1 million.
The Department of Health and Human Services had projected 3.3 million people would sign up by Dec. 31.
Administration officials say they remain optimistic they'll reach the Congressional Budget Office's projection of 7 million signups by March 31, even if signups by Dec. 31 fall short.
Consumers must pay the first month's premium before coverage takes effect.
America's Health Insurance Plans, the industry's largest trade group, says its members will give consumers until Jan. 10 to pay their January premium, and get coverage retroactive to Jan. 1, provided they choose a plan by Tuesday's deadline.
Some state-based exchanges are maintaining a Dec. 31 deadline for making the first month's payment.
Healthcare advocates recommend if people want to be sure they're covered in January, they should pay the first month's premium by Dec. 31, unless they can confirm they have the 10-day grace period for making the first month's premium.