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.

Through whom are repentance and forgiveness granted?
How had Christ's teaching by parables been foretold?
What question must every unfaithful parent meet?
Who was the seed here referred to?
When are the saints to he like Jesus?
What title is given the last book of the Bible?
What takes place under the sixth plague?

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