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 was foretold of His treatment while on the cross?
What apostasy from the worship of God is named in this message?
By what is the knowledge of sin?
What part of the law of God especially has the Papacy thought to change?
On what day does John say he was in the Spirit?
What did Christ say of worship based upon the commandments of men?
When we see all these things, what are we to know?

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