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 description of the Apostle Paul's experience is in harmony with this?
How complete should this obedience be?
21. Although Pilate had declared his belief in Christ's innocence, yet what cruel punishment did he inflict upon Him?
Upon whom is God's fury to be poured?
Who does God say shall die?
When and by whom was the Sabbath made?
Will this he a time of judgment?

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