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.

Upon what basis has Christ taught us to ask forgiveness?
How does the death of Christ bear testimony to this?
How closely are righteousness and life thus united?
Who opens the judgment and presides over it?
What immediately followed this complete apostasy?
What was the date of Constantine's famous Sunday law?
In ministering to the needy, whom are we really serving?

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