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.

Who is the Comforter, and what was He to do?
Whom did Christ say the Holy Spirit would glorify?
What is said of the fourth kingdom?
What will He then do for each one?
How many will yield to the demand to worship the beast?
Why did the Lord destroy Sodom?
Why did Christ say the meek are blessed?

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