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 promise is made to those who repent and are baptized ?
Where was Christ to be born?
11. How did the Jewish leaders in the days of Christ, for the sake of gain, make void the fifth commandment?
What is the Lord able to do regarding the future?
What did Jesus say of His attitude toward the law?
When was the papal head of the first beast of Revelation 13 wounded?
How great reward has God promised those that love Him?

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