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.

How did some of the believers at Ephesus testify to the sincerity of the confession of their sins?
In what working of God is faith to be exercised in connection with baptism?
What are children declared to be?
What else does the prophecy say the little horn would do?
What example did Christ set in Sabbath observance?
When the state enacts religious laws, what is effected?
What did Christ say of the certainty of this prophecy?

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