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 rendered it possible for God to treat sinners thus?
What danger attends the teaching of false doctrine?
What is said of Christ's childhood and early life?
What was denoted by the ten horns?
What change did Daniel see take place in these horns?
For what did Christ reprove the Pharisees?
In the judgment, how many angels minister before God?

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