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 God's design in thus giving the Scriptures?
When Pilate heard that Christ was from Galilee, what did he do?
Who was the last Babylonian king?
What was Christ's reply?
How did Christ speak of the gathering of the Gentiles?
How fully has God promised to supply our needs?
What will His true followers do?

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