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 does God's character preclude Him from doing?
What is said of the strength and wisdom of God?
What ordinance is closely associated with believing the gospel ?
What great gift comes with our acceptance of Christ?
How does the Spirit witness to our acceptance with God?
Under the old covenant, what did the people promise?
Why did God command Israel to hallow the Sabbath?

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