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.

When may we find acceptance with God through Christ?
In all we do, whose glory should we have in view?
What will be the fate of those who do not obey the gospel of Christ?
Where was Christ to be born?
How did Solomon's parents regard him as a child?
To whom is salvation promised at Christ's appearing?
What divine law of retroaction attends giving?

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