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 application did Jesus make of this prophecy?
13. What did John the Baptist say to the Pharisees and Sadducees when he saw them come to his baptism?
6. Under the influence of her tender care and faithful instruction, what is said of the child life of Jesus?
By what means was the image broken to pieces?
When and by whom was the Council of Nice convened?
Following the signs of His coming, what did Christ say would take place?
With what are the twelve foundations adorned?

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