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 writings are specifically mentioned as containing some things difficult to understand?
What are some of the attributes of God?
What change is wrought in conversion, or the new birth?
What change is thus brought about?
How did Jesus honor His parents?
Is there any evidence that such an image will be made?
2. What sign had been foretold by the prophet Isaiah by which Christ, at His first advent, might be known as the Messiah?

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