The deadline has been extended by a day for those who want to receive health insurance coverage through the federal exchange beginning Jan. 1, 2014.
In a statement, Julie Bataille, director of Centers for Medicare & Medicaid Services' Office of Communications said, "We recognize that many have chosen to make their final decisions on today's deadline and we are committed to making sure they can do so."
Early Monday, an alert on the Healthcare.gov website warned users that high demand could cause some delays.
"Anticipating high demand and the fact that consumers may be enrolling from multiple time zones, we have taken steps to make sure that those who select a plan through tomorrow will get coverage for Jan 1," said Bataille.
Residents in D.C. and Virginia have until midnight Monday to sign up while those in Maryland have until Dec. 27. Thirty-six states, including Virginia, rely on the federal website for enrollment. The remaining 14 states and the District of Columbia have their own exchange websites.