WASHINGTON — For most Americans, Monday is the deadline to sign up for health insurance that takes effect on Jan. 1. It was supposed to be a turning point in the troubled history of the new health care law, the moment when the spotlight would shift from the federal government’s online marketplace to the insurance companies providing coverage to hundreds of thousands and eventually millions of people.

But as the date approaches, a series of decisions by the Obama administration to delay some of the law’s most important provisions and to extend some deadlines has caused uncertainty among insurers and confusion among consumers.

Already the enrollment of young adults appears to be lagging behind the expectations of federal officials and insurers. And insurers, struggling with problems caused by the chaotic debut of the federal insurance exchange in October, have criticized the administration’s last-minute changes, saying they could cause instability in the market.

The sudden shifts have sent a mixed message to consumers about the significance of the Monday deadline. They reduce the sense of urgency but not the need for coverage. Accordingly, federal and state officials said they were prepared for an increase in activity on their websites and at telephone call centers as uninsured consumers rushed to beat the deadline.