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.

Unto whom will the saints forever ascribe praise?
Because some thought He had come to destroy the law, what did Christ say?
For what purpose were these gifts bestowed upon the church?
What example did Christ set in Sabbath observance?
What was to he inflicted upon one of the heads of this beast?
What role do his agents assume?
What was shown the Apostle John in vision?

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