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 the result of godly sorrow?
What was the Comforter to do when He came?
And how should husbands regard their wives?
Who will thus he judged by the saints?
What is declared to be the chief advantage possessed by the Jews?
Are angels of a higher order of beings than man?
12. What class of God's servants have evil angels, through wicked men, seemed to make special objects of attack?

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