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 are some of the attributes of God?
In what words is His impartiality proclaimed?
In what way was Jonah a type of Christ?
What cheering message has Christ sent to His people touching His resurrection?
How was the overthrow of ancient Babylon foretold?
How comprehensive are these commandments?
How is the law further described?

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