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 ordinance is closely associated with believing the gospel ?
What deliverance was thus made possible?
With what did He say they would he baptized?
Why was it necessary to make this atonement?
How did the Apostle Peter say the message of the Lord's coming would be treated by some?
Because of disobedience, what experience came to Israel ?
What did Christ say would be the experience of His people in this world?

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