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 doctrines are some to be misled in the last days?
What saved the people from the predicted overthrow?
To what insults was Christ subjected at the house of the high priest?
How faithfully should parents teach their children the Word of God?
How are the nations gathered to the battle of Armageddon?
How long will they sleep there?
How long will they live?

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