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, therefore, must he the character of His Word?
What Christian ordinance has been given as a memorial of Christ's burial and resurrection?
Who is responsible for the present state Sunday laws of the United States?
Under what authority were its decrees published?
What has been promised in order that man may be redeemed from this condition?
What special blessings were conferred on the Israelites?
At the pouring out of the seventh plague, what announcement will be made?

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