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 comment has the Apostle Paul made upon this commandment?
2. After being threatened with death if they did not make known the dream and the interpretation, what did the wise men say to the king?
How is the church of the last days described?
To whom was this covenant-promise later renewed?
What did John say was to be the end of this beast?
Who alone knows the exact day of Christ's coming?
22. While many will be deceived by these wonders, and accept of the false christs that appear, what will those say who have maintained their love for the truth, and patiently waited for Christ's return?

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