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.

Where in the Bible do we find the first prom Redeemer?
What promise is made to those who repent and are baptized ?
What effect did this dream have upon Daniel?
What will He say to those on the left?
What definite period is mentioned under this trumpet?
Through whom will come this redemption from the grave?
What special blessings were conferred on the Israelites?

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