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.

How complete was Christ's victory over death?
Why cannot the world receive Him?
Why was woe at this same time proclaimed to the world?
Seeking a fair remuneration, what do many laborers do?
What striking figure is used to describe the destruction wrought under the second trumpet?
What special blessings were conferred on the Israelites?
4. And when the new heavens and the new earth have appeared, and the holy city, New Jerusalem, then he made?

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