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 contrast is drawn in the Scriptures between the Creator and false gods?
What is said of his mother and his grandmother?
Were any efforts to he made to reunite the divided empire of Rome?
Who is this dragon?
What power mentioned in the thirteenth chapter of Revelation is to enforce this mark?
What plain proposition did he submit to all Israel?
What was the character of Christ's work among men?

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