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 reply did Christ make?
Concerning what subject ought we to be informed?
What effect did this dream have upon Daniel?
By what was the third universal empire symbolized?
What provision has been made so that we may keep God's law?
What will the Lord he to His people at this time?
Referring to Peter's coming sore trial, for what did Christ say He had prayed?

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