How did this change in observance of days come about, suddenly or gradually?

Answer

Gradually.
NOTES - "The Christian church made no formal, but a gradual and almost unconscious
transference of the one day to the other." 'The Voice From Sinai," by Archdeacon W. F. Farrar page 167.
This of itself is evidence that there was no divine command for the change of the Sabbath.

What did these four beasts represent?
In what other prophecies is this same period mentioned?
Was the ceremonial law a complete law in itself?
Is any of this kind of work to be done on the Sabbath?
When we see all these things, what are we to know?
Who is it that blinds men to the gospel of Christ?
Who will be permitted to enter it?

Questions & Answers are from the book Bible Readings for the Home Circle