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 whom were the men directed who thus spoke for God?
Who alone comprehends the things of God?
What leads sinners to repentance?
What is the measure of the greatness of God's mercy?
With what words did the chief priests and others mock Jesus while on the cross?
What accompanies the earthquake?
What is Spiritualism defined to be?

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