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.

For what purpose were the Scriptures written?
What is felt in heaven when a sinner repents?
At what cost did even He learn the lesson of obedience?
2. What attitude of rivalry was the Papacy, represented by the little horn, to assume toward the Most High?
What did this little horn do to the people of God?
What does He indicate as one of its purposes?
Do the Scriptures indicate that there will he violence manifested in this conflict?

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