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 remarkable experience attended the baptism of Jesus?
How does the death of Christ bear testimony to this?
Had the Lord spoken regarding the Sabbath previous to this time?
How does Paul describe this dedication of the covenant?
8. Did God bless and sanctify the seventh day while He was resting upon it, or when His rest on that day was past?
At His ascension, what assurance was given of Christs return ?
By what are we to try them?

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