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.

At the beginning of His ministry, what example did Jesus set for the benefit of His followers?
How did the aged prophetess Anna express herself at the sight of Jesus?
What is the great secret of a happy home?
Whom did Christ indicate as fulfilling this prophecy?
How is the closing period of tribulation of the church during this time referred to?
What promise is made to those that mourn?
How much of the land of Canaan did Abraham own in his lifetime?

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