As the Dec. 15 deadline for signing up for health insurance under Obamacare closes, states are reporting an increase in their enrollment numbers, and some have decided to give consumers more time to pick a health plan. Although Americans have until Feb. 15 to enroll without paying a tax penalty, any applications completed in most states after 3 a.m. – or midnight Pacific time Dec. 15 – will not kick in Jan. 1, 2015, but will instead activated during the following month.

“For the uninsured – many of whom have gone for years without coverage – that means this is the last opportunity to sign up in time to start the new year with the peace of mind that comes with health insurance,” says Anne Filipic, president of Enroll America, a non-profit grassroots organization that helps inform and enroll Americans in health plans.

Under the Affordable Care Act, every American must purchase health insurance or pay a tax penalty. Most Americans have coverage through employers, while seniors are covered by Medicare and low-income residents are covered by Medicaid. Those who fall into the gap – such as small-business owners or self-employed Americans – can buy private, government-subsidized health insurance through online marketplaces or exchanges.

Healthcare.gov isn't the site every American can use – there are separate websites for the District of Columbia and 13 states. Open enrollment, the three-month period during which people can sign up for insurance, began Nov. 15 and will last until Feb. 15. There are some variations in deadlines by state. New York, for instance, has extended its deadline to Dec. 20. Other states announced extensions late Monday.

With some exceptions, most people who enrolled in the marketplaces last year will be automatically re-enrolled in coverage by Monday, and those who do not make changes to their old plan likely will see increases in their premium costs. About 70 percent of those who take the time to shop again on the marketplaces could get a cheaper deal, according to the Department of Health and Human Services.