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.

How complete should this obedience be?
Whose example are we urged to imitate?
What is said of those who read, hear, and keep the things contained in this book?
What specifications of \"the man of sin\" are thus fully met?
What was the Lord's covenant which they were to keep as their part of this covenant?
What did God do in the beginning on the seventh day?
When are the righteous to be recompensed?

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