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 does the sorrow of the world do?
How complete a Saviour is He?
In what spirit should children obey their parents?
What do the works of God declare?
What will be the third plague?
Do they take any part in earthly things?
What did John see concerning this city?

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