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.

5. What further statement of Christ seems to lay the responsibility for the origin of sin upon Satan and his angels?
How was Christ to be received by His own people?
What is revealed by the law?
Is it the duty of gospel ministers to execute vengeance?
Of what is the Sabbath commandment a part?
Does the Papacy acknowledge that it has changed the Sabbath?
What did Paul say regarding our duty to the poor?

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