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 led to Satan's sin, rebellion, and downfall?
How thoroughly will the effects of sin he removed?
By what doctrines are some to be misled in the last days?
In symbolic prophecy what length of time is represented by a day?
How does the Lord, through the prophet Isaiah, indicate what is true Sabbath-keeping?
What promise has God made to bereaved mothers?
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