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.

How enduring is Gods love for us?
How did the people of Samaria publicly testify to their faith in the preaching of Philip?
What proves that the law is an undivided whole?
18. In what statement was Christ promised as a Savior and Deliverer of the race as soon as sin entered?
On what day did Paul and his companions preach to the devout women at Phillippi?
To whom is salvation promised at Christ's appearing?
By what name is the fifth state of the church addressed?

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