National Sunday law involves a conspiracy theory which alleges that the United States government is on the verge of enacting a national blue law that would make Sunday a day of rest and worship. The theory depends on the idea that the Pope is the Antichrist and the Mark of the Beast is worshipped on Sunday. Sinister forces (read: the Vatican) are conspiring to enact a national Sunday law in the United States, which would be the trigger that unleashes the fulfillment of the prophecies found in the Biblical books of Daniel and Revelation. In addition, this law would outlaw worshipping on Saturday, thus beginning a period of persecution of those who worship on Saturday, or Sabbath.
This idea originated in the 19th century within Seventh-day Adventism (which regards the Sabbath as Saturday), and some on the fringes of the SDA church have taken a handful of failed Congressional bills and papal writings and inflated them into the trigger of the apocalypse. This is quite ironic considering that you would think that opposition to blue laws might come from more secular groups, yet infighting[1] and schism aren't simply conjectural