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 was He treated?
Who opens the judgment and presides over it?
How long will the saints engage in this work of judgment?
What is said concerning the captivity and downfall of the Papacy?
15. Then when miracles are performed by spirits purporting to be those of our dead friends, to what shall we attribute them?
What is Jesus able to do?
What did John see concerning this city?

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