CHICAGO - Anticipating heavy traffic on the government's health-care website, the Obama administration extended yesterday's deadline for signing up for insurance by a day, giving Americans in 36 states more time to select a plan.

It was the latest in a series of pushed-back deadlines and delays that have marked the rollout of the health-care law.

But federal officials urged buyers not to procrastinate.

"You should not wait until tomorrow. If you are aiming to get coverage Jan. 1, you should try to sign up today," said Julie Bataille, a spokeswoman for the federal agency in charge of the overhaul.

Bataille said the grace period - which runs through today - was being offered to accommodate people from different time zones and to allow for any technical problems that might result from a last-minute rush of applicants.

The HealthCare.gov site had a disastrous, glitch-prone debut in October, but the administration said the system was running well yesterday.

By the afternoon, the site had received a record 850,000 visits, five times the number logged by the same time last Monday, the administration said.

The Obama administration is hoping for a surge of year-end enrollments to show that the technical problems were merely a temporary setback. That would also go a long way toward easing concerns that insurance companies won't be able to sign up enough young, healthy people to keep prices low for everyone.

But the grace period may have been a tacit acknowledgment that the website remains vulnerable to heavy traffic. What's more, the delay offered critics another opportunity to argue that the law still isn't working and that President Obama keeps changing the rules.

Yesterday had been the deadline for Americans in the 36 states served by the federal site to sign up if they wanted coverage at the start of the new year. The remaining states operate their own online marketplaces, and some of them have also extended their deadlines.