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.

Through whom is this reconciliation made?
For what is all Scripture profitable?
What will sound doctrine enable the faithful teacher to do?
By what are all men to be finally judged?
After the law had been proclaimed from Sinai, what did the people again say?
Where did he say he would wait?
What work constitutes the fast most acceptable to God?

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