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 charge did Jesus bring against the Pharisees?
In the visions of John, what interpretation is given to the woman who sat upon many waters?
How are those described who will he prepared for the coming of Christ?
What was to take place under the third trumpet?
How does Satan deceive the people?
What is Jesus able to do?
What is the measurement of the city?

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