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.

By whom were the men directed who thus spoke for God?
What is the foundation of faith?
What event does he speak of as immediately following these times of refreshing?
After she became queen of Persia, how did Esther show her integrity to God?
What change will then take place in both the living and the sleeping saints?
14. What scripture for ever precludes the idea that the dead come back to earth to communicate with the living?
What precious promise has God made to His children?

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