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.

14. Within less than forty days after the making of this covenant, while Moses tarried in the mount, what did the people say to Aaron?
How did Jesus emphasize the necessity of conversion?
Of what sanctuary, or tabernacle is Christ the minister?
Whom did Jesus pronounce blessed?
How long will they reign?
Who is responsible for the present state Sunday laws of the United States?
What are God's people told to do at this time?

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