I think it would be the last day of the United States of America. Some people would perhaps say it still existed afterwords but in reality the country would tear itself apart almost overnight. Every city would burn, sanctions would be levied against the country by foreign powers and the economy would collapse and without an economy, the government would collapse. I would then expect foreign involvment. UN peace keepers, drone strikes. Special forces being deployed to arm and train partisans, advanced weapons deliveries to local militia groups. Everything that we have done in places like Bosnia, Syria, Iraq, but happening here. The country could end up divided up between Russia and China and EU, maybe with Mexico and South America taking back the southern regions.
My reaction would be to align myself with whatever local warlord emerged who seemed to offer the best deal and fight to secure my area against the chaos and hope when it was over we where somehow able to retain something resembling normal life in whatever balkanized territory emerged.
Without knowing the 'why' The why of course, could have a huge effect.
Because the reaction would be so extreme, I view the likelihood of mass overnight arrests in this country as extremely low. Americans have a very low tolerance for those kind of actions and with people so stressed, the reaction would be severe.