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.

At His birth, what message did the angel bring to the shepherds abiding in the field?
Who moved upon the prophets to give their messages?
13. In this same prophecy, how is the Church of Rome, the Papacy, designated as the antitype of ancient Babylon?
What did they have in the law?
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?
What did Charlemagne's Sunday law of 800 require?
When did David say he would be satisfied?

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