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 long will the Word of God endure?
What other prediction asserts the same great truth?
What classes of persons are necessarily shut out of the kingdom of God?
What did Isaiah say Christ would do with the law?
What does He indicate as one of its purposes?
After accepting Christ, what are we to do?
For what purpose was man made?

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