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 what principle is genuine faith actuated?
Through whom is this reconciliation made?
By what commands has God guarded the marriage relation?
To what did Gabriel now direct Daniel's attention?
How was the \"devotion\", of the \"faithful\" disturbed?
How long will they live?
How may grace and peace be multiplied in believers?

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