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.

For what purpose were the Scriptures written?
What question was asked in the hearing of the prophet?
Why did God command Israel to hallow the Sabbath?
Was there anything made without Christ?
What other event is mentioned under this seal?
Who is to reign finally over God's kingdom?
In His transfiguration, how did Christ appear?

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