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 place has the will in determining whether man shall have life?
What will follow this union with Christ in His death and resurrection?
How does the Spirit witness to our acceptance with God?
What is the very life of the Spirit?
What was the result of this warfare by means of \"fire and smoke and brimstone\"?
What does Christ promise overcomers?
In ministering to the needy, whom are we really serving?

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