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 did the father show his joy at his son's return?
Whom did Christ say the Holy Spirit would glorify?
To whom was Belshazzar's kingdom given?
How extensively is this message to he proclaimed?
Who did He say would enter the kingdom of heaven?
What promise did Christ make concerning His coming?
What announcement is made under the seventh plague?

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