The one-day grace period was just the latest in a string of delays and reversals, and critics of President Barack Obama's signature program seized on it as more evidence that the overhaul is in trouble.
"The amazing, ever-expanding deadline? It's clearly a sign of desperation by the administration to do everything they can to increase the number of people signing up," said health economist Gail Wilensky, who ran Medicare for President George H.W. Bush.
The website went through extensive hardware and software upgrades to make it more reliable and increase its capacity.
When the number of simultaneous users reached 60,000 on Monday, site operators employed a queuing system that allows people to either wait or give an email address to be invited back later, the government said. More than 129,000 users gave their email.
On Tuesday, traffic wasn't heavy enough to trigger the system, McGuinness said in the afternoon.
Many states operate their own online marketplaces for buying coverage, and some of them also extended their deadlines.
The insurance industry, too, has pushed back deadlines for payment, with most health plans allowing customers to pay by Jan. 10 and still get coverage retroactive to Jan. 1.
"With deadlines that keep changing, insurers want to alleviate confusion," said Robert Zirkelbach, spokesman for America's Health Insurance Plans. "Health plans are going to do everything they can to help consumers with the enrollment process."
Obama said late last week that more than 1 million Americans had enrolled for coverage since Oct. 1.
The administration's estimates call for 3.3 million to sign up by Dec. 31, and the target is 7 million by the end of March. After that, people who fail to buy coverage can face tax penalties.
Associated Press Medical Writer Carla K. Johnson can be reached at http://www.twitter.com/CarlaKJohnson