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.

To what knowledge would God have all men come?
Because some thought He had come to destroy the law, what did Christ say?
Where did Moses command the Levites to put the book of the law which he had written?
What title is given the last book of the Bible?
What scene in heaven was presented to the prophet as the seventh trumpet was about to sound?
At this time what event is imminent?
Why does God comfort us in tribulation?

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