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 prophecy foretold Christ's triumph over death?
What is the gift of God?
How did Christ suggest that His disciples follow His example in teaching gospel truth?
At His ascension, what assurance was given of Christs return ?
What will be the fourth plague?
Before Gentiles become Israelites, in what condition are they?
Where is this everlasting or eternal life?

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