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 home did God make for man in the beginning?
With what was He filled?
How does Paul again express this same truth?
What definite period is mentioned under this trumpet?
Who witnessed the laying of the foundations of the earth?
With what were many possessed in Christ's time?
Why are its gates not to be closed?

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