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.

In what words does Daniel foretell Christ's receiving His kingdom?
What home did God make for man in the beginning?
In whom has God made us accepted?
Who was represented by \"the great horn,\" and what followed when it was broken?
4. What was another memorial instituted to commemorate another signal providence in behalf of the Israelites?
Of what besides a knowledge of God as Creator, is the Sabbath a sign?
What day does the commandment say is the Lord's?

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