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 assured to the believer in Christ?
Who moved upon the prophets to give their messages?
What is said of the little horn as compared with the ten horns of the fourth beast of Daniel 7?
What did the goat with the notable horn represent?
Why did God destroy the antediluvian world?
How does Paul speak of this coming?
Under whose influence are the nations to be assembled?

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