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.

In what act especially has Gods love been manifest?
How early did Timothy know the Scriptures?
8. Did God bless and sanctify the seventh day while He was resting upon it, or when His rest on that day was past?
12. Because of Christ's healing a woman of an infirmity on the Sabbath, what did the ruler of a certain synagogue say?
Then by whom was the Sabbath made?
For what purpose did Christ say He would come again?
What does the Psalmist say about the dead praising God?

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