On March 23, 2010, President Obama signed the Affordable Care Act into law. A few minutes later, lawsuits were filed–first in Florida, and shortly thereafter in Virginia–challenging the law’s constitutionality. The rest is history (in my book).
In truth, the actual anniversary of the law should be March 30, 2010, when the President signed th Health Care and Education Reconciliation Act of 2010, which is the law that actually made the ACA count.