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.

By what means is sin made known?
What change is wrought by beholding Jesus?
What testimony did the chief priests and Pharisees bear concerning Christ's work?
What is said of the stability of God's character?
What statement did Paul make concerning his faith and manner of worship?
Through whom did God create the world?
To whom is salvation promised at Christ's appearing?

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