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 gave special force to the oath of an angel?
In what words is His impartiality proclaimed?
After one truly receives Christ, whose life will be manifested in him?
What was the fourth beast declared to be?
What assurance was given to Daniel concerning the period of time mentioned in verse 14?
What does Christ say about our duty to the state?
How may one grow in grace?

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