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 ordinance is closely associated with believing the gospel ?
How did persecution affect the preaching of the gospel?
In what spirit should children obey their parents?
What question must every unfaithful parent meet?
What did the people once again promise to do?
To what length will this effort to enforce the worship of the image of the beast be carried?
What was already at work in the church in Paul's day?

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