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 does the sorrow of the world do?
What divine purpose was wrought out in the sufferings of Christ?
10. After the fall, what sort of marriages were introduced by men, which were productive of great evil?
How should our children be taught?
What did God say to Abraham concerning the Amorites?
What was Christ's custom respecting the Sabbath?
What imperial law was issued in AD. 386?

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