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 encouraging statement follows concerning the supply of power to the faint?
What effect did the preaching of the Apostle Paul have upon the Gentiles?
When will the gifts of the Spirit he no longer needed?
In symbolic prophecy what length of time is represented by a day?
How enduring is the moral law?
What two covenants are contrasted in the Bible?
When will immortality be conferred upon the saints?

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