The Trump administration plans to shut down healthcare.gov, a website consumers use to sign up for Affordable Care Act insurance, for 12 hours on nearly every Sunday during the coming ACA enrollment season.

The outages, which the administration says are needed for maintenance, will occur from midnight through noon on every Sunday other than Dec. 10. This year’s enrollment season—which the administration has shortened from three months to six weeks—will run from Nov. 1 through Dec. 15 for the 38 states that use the federal...