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 is proved by any attempt to be justified by the law?
What did Babylon give to the nations to drink?
How may we be freed from the guilt of our sins, or our transgressions of God's law?
What reason is given for making the new covenant?
What testimony did Christ bear concerning His relation to the law and the prophets?
What will He say to those on His right hand?
Why did the unjust judge answer the widow's prayer?

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