So, in what conditions, would a National Sunday Law be enforced?
Sunday laws – or "Blue laws", as they're often called in the United Stated – used to be widespread and are slowly, piece by piece, being repealed. Usually, they are industry specific. For example, until recently in Colorado, you could not sell package liquor on Sundays (now you can). But, you still can't buy a car on a Sunday.
It also used to be against the law to open a bank on a Sunday, something that is now legal, although only a few banks do so (TCF is one of the few).
New Zealand used to have fairy strict Blue law, and Germany is probably the world leader for the intense blue law enforcement across a wide variety of industries at once, although every blue law has some exceptions. Israel has some Sabbath Blue Laws.
Usually enforcement is not a problem because violating the law would be a ground for violating a business license.
Can we guess what would be the steps to such a change?
There won't be such a change because the trend is going strongly the other direction. Quite a few were passed in the Eisenhower Administration coincident with adding "under God" to the Pledge of Allegiance, "In God We Trust" on currency, putting Ten Commandments plaques in front of municipal buildings (a project of the Knights of Columbus, a Roman Catholic civic organization), and union efforts to secure time off for workers (hence the union slogan "we brought you the weekend") at a time of extreme labor power and a shortage of workers in the manufacturing industry.
This surge of religious sentiment was a product of the Cold War. It distinguished the United States as a religious country from the Soviet Union and its allies that were atheist as a matter of state edict.
How long it will take to be introduced (since we find about it and
until the religion freedom will be limited to people)?
It won't happen at all. It peaked most recently in the 1950s (although there were previous peaks) and has declined pretty steadily since the 1970s as they have been repealed piecemeal, although the Courts have upheld Blue Laws as a constitutional way to provide a coordinated break from work to people.
Will people oppose to such a law?
Yes. People these days don't like to be inconvenienced and church attendance and religious identification rates are both falling dramatically.
But, people also opposed repealing, for example, the Sunday liquor store ban in Colorado. The opposition came mostly from family owned liquor stores where the owner would either face competitive pressure to work seven days a week, or would have to hire a first employee so as not to have to work seven days a week despite the competitive pressure, increasing their overhead.
The store owners, probably accurately, guessed that their weekly sales wouldn't change much whether they were open six days a week or seven, because aggregate demand wouldn't change, but they would have to spend more days working, and if they weren't open all seven days, they would lose market share to those who were, so they needed the government to honor this market collusion to close one day a week.
I'm sure it's not going to say directly: if you don't go to the church
on Sundays, you'll die, but good arguments will be eventually found.
Maybe, will it be an executive order?
Historically, this has almost never happened. When you pass a law like that legislator's want credit for it with the base of people who care.
The Puritans in New England did formally give a religious justification as that continued until the First Amendment's establishment clause jurisprudence was well established well into the 1800s.
But the argument that there is a secular purpose for Blue Laws is much more legitimate than the argument that there is a secular purpose for "In God We Trust" on coins which Courts have nonetheless accepted (under the "ceremonial deism" doctrine). A national day of rest on Sundays isn't that much different from, for example, laws that declare that the 4th of July is a national holiday which no one would claim violates the First Amendment's establishment clause.
What would it happen if the president would sign tomorrow an executive
order which will enforce the Sunday keeping? Are there any What are
the ways to stop it?
First of all, the President wouldn't sign one. Secondly, I can't think of any statute that would give him the authority to issue one. If he tried to do so, somebody materially affected by the law, such as a business forced to close its doors on Sunday, would go to a District Court judge and seek first a temporary restraining order, then a preliminary injunction and ultimately a permanent injunction establishing that he has no statutory authority to do so.
It doesn't have to last long since, in the view of the 7th Day
Adventists, it will not take a long time since the Sunday worship
enforcement and the second coming of Jesus.
If you want to believe that, so be it.
This belief sounds like conspiratorial crazy talk to those of us who are not members of the faith, and people have been predicting the second coming of Jesus is right around the corner since the day after the crucifixion. But, given the utterly unlikelihood of a Blue Law every being enacted again, you have nothing to worry about.
(As an aside, why do Christians call it the "second coming of Jesus" instead of the "third coming of Jesus"? The guy is born (first time), and then dies and comes back and the ascends ("second time"), so why wouldn't a future return to Earth be a third coming? I never really understood that phrase.)