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 scripture shows that the Roman emperors ruled the world?
Was the judgment still future in Paul's day?
Is there any danger of God's chosen people forgetting Him?
Hearing these words, what questions did the disciples ask?
What title is given the last book of the Bible?
16. After the sealing work brought to view in Revelation 7, which takes place under the sixth seal, how is the seventh seal introduced?
Whose names are in the foundations of the holy city?

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