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.

How does godly sorrow for sin manifest itself?
What question did -Christ's wonderful teaching call forth?
What is said of his mother and his grandmother?
Over what do the people of God finally gain the victory?
What, according to the prophecy of Daniel, was to characterize the time of the end?
What promise is made to those who look for Him?
Why did the unjust judge answer the widow's prayer?

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