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.

Where in the Bible do we find the first prom Redeemer?
For what are we told to pray at this time?
How did He show that He loved them?
What was Messiah to do during the seventieth week?
By what other terms are these covenants designated?
Who made the Sabbath?
Why are we exhorted to patience?

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