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.

How completely does God wish to separate sin from us?
What did the devout Simeon say when he saw the child Jesus?
What place has the Sabbath in the work of salvation?
16. What striking testimony is borne by Neander, the noted church historian, regarding the origin of the Sunday Sabbath?
Against whom do we wrestle?
6. Looking forward to the conflicts through which His followers must pass, what cheering message did Christ send them through the Apostle John?
How did Elijah pray before obtaining his request?

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