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 instruction shows that sanctification is a progressive work?
At what cost did even He learn the lesson of obedience?
Why should husbands be considerate of their wives?
What call was made to come out of Babylon?
What warlike scene is given under this trumpet?
What is one good evidence of genuine repentance?
To whom has God given the earth?

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