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 kinds of disease and sickness did Jesus cure?
What else does the prophecy say the little horn would do?
What is declared to be the chief advantage possessed by the Jews?
12. Are men's hearts now \"failing them for fear, and for looking after those things which are coming on the earth\"?
22. When surrounded by the Syrian host, what did Elisha say, and for what did he pray, to encourage his frightened servant?
What shows that the actions of men are recorded?
Where will all the saints then go?

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