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.

Will sin and its evil results ever appear again?
In what scripture does Christ apply the same emblem to Himself ?
As He cried out in agony on the cross, and said, 9, thirst,\" what was given Him?
What question was asked in the hearing of the prophet?
What is the nature of Gods law?
What title is given the last book of the Bible?
Why is it better to go to the house of mourning than to the house of feasting?

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