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 effect does obedience to the truth have?
With what words did Daniel begin the interpretation of the dream?
How enduring are His commandments?
How was the \"devotion\", of the \"faithful\" disturbed?
Why was the gospel to be preached to the Gentiles?
What was the color of the symbol under the third seal?
Is Christ ever called an angel?

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