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 is sin declared to be?
How then do all become children of God?
What does the prophet Jeremiah declare Christ to be?
How did Christ show His tender regard for children?
What did this nearness of the time of restoration from captivity lead Daniel to do?
In the visions of John, what interpretation is given to the woman who sat upon many waters?
10. Because the disciples plucked a few ears of corn on the Sabbath day to satisfy hunger, what accusation did the Pharisees make against them to Christ?

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