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.

22. After being united with Christ in the likeness of His death and resurrection, what should the believer do?
What assurance has the believer of his union with God?
Why was Christ's preaching so impressive?
Where will Christ then take His people?
What message announces the judgment hour come?
4. What explanation did Moses give the rulers of Israel concerning the withholding of the manna on the seventh day in the wilderness of Sin, before they reached Sinai?
What is said concerning the captivity and downfall of the Papacy?

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