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 was predicted of Christ's preaching?
Upon what basis has Christ taught us to ask forgiveness?
When Christ ascended, what did He give to men?
Who did He say would be saved?
What, according to the prophecy of Daniel, was to characterize the time of the end?
Whatever may come, what blessed assurance has everyone who loves God?
What work constitutes the fast most acceptable to God?

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