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.

Where was the Messiah to he born?
Is there to be a special message for the last days?
What is sin declared to be?
How is sin defined?
At Corinth what did he do on the Sabbath days?
What has always been the result of religious legislation or a union of church a state ?
To whom do those things which are revealed belong?

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