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 is the evidence of genuine, living faith?
What does Christ declare Himself to be?
Upon what ground were the previous promises then renewed to Abraham?
What are children declared to be?
What was done with the blood of the goat upon which the Lord's lot fell?
What test case arose in the time of the apostles over this question?
Who will be permitted to enter it?

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