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.

19. What message did Christ send to John the Baptist while John was in prison, to strengthen his wavering faith?
With what words did Daniel begin the interpretation of the dream?
Why did God make His promise to the seed of Abraham?
What besides creation were Israel to remember when they kept the Sabbath?
What is first commanded in the Sabbath commandment?
16. After the sealing work brought to view in Revelation 7, which takes place under the sixth seal, how is the seventh seal introduced?
19. When this city becomes the metropolis of the new earth, what will be the condition of God's people?

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