ObamaCare's main enrollment site is facing its first big deadline since relaunching last month, a crucial test for a system that will help define the law's performance in its second full year of implementation. People shopping for health insurance must sign up by Monday at midnight if they want their coverage to start Jan. 1. HealthCare.gov appeared to be enrolling users successfully Monday morning and over the weekend, a good sign with user traffic expected to rise significantly. The Obama administration is engaged in a full-court press to encourage people to visit the site prior to Monday's deadline.