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.

To what extent has God magnified His Word?
With what words do the Scriptures conclude the record of Christ's early life?
What lesson did Christ design to teach in healing the man sick of the palsy?
To whom does God reveal the secrets of the future?
What question was asked in the vision of Daniel 8?
What did the bishops determine to do?
How do evil spirits sometimes treat those thus possessed ?

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