Health coverage after losing my job: marketplace or Medicaid? (update)

I have 12 weeks left on my current insurance and I am comparing options. This is my first time dealing with any of this. I do not want a gap in coverage. How fast did coverage start?

Losing job-based coverage usually opens a special enrolment window, so check the deadline on the official marketplace site. I missed it by a day once and regretted it.