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 chapter in the Bible is devoted to faith?
What are some good fruits of proper child training?
What did Daniel say the king had seen in his dream?
In the interpretation of the vision, how is this self exaltation set forth?
How do those with renewed hearts and minds regard the commandments of God?
Why did God destroy the antediluvian world?
What promise was made to Abraham concerning the land?

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