The terms of your question should be inverted; for it is not Sunday that is called the first day of the week, but the first day of the week that is called Sunday. The number was before the name.
Then when the question is inverted, and it is asked what Bible authority we have for calling the first day of the week Sunday, the reply must be that we have none.
The following questions, “When did they change the Sabbath from the seventh day of the week to the first day? Why did they do so?” were sent to the New York Tribune, by a reader of that journal. The brief answer, which follows, was given in that journal, and is very clear and explicit. We reprint it in hope that it may help others who have similar queries, and stimulate them to further investigation.
"Was the cleansing of the sanctuary finished at the end of the twenty-three hundred days? Or is the work of cleansing still going on? If it is still going on, when will it be finished?"