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.

How should the truth ever be cherished?
What was the fourth beast declared to be?
What prophecy spoke of their return from captivity?
What was the color of the symbol under the third seal?
What was the result of this warfare by means of \"fire and smoke and brimstone\"?
To whose authority are the angels subject?
Whatever may come, what blessed assurance has everyone who loves God?

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