What is the Adventist response to Justin Martyr? Justin Martyr described Christians worshiping on Sunday twice in his first apology.

And on the day called Sunday, all who live in cities or in the country gather together to one place
But Sunday is the day on which we all hold our common assembly.

This document cannot be dated any later than 165 since the author died at 165.
In the great controversy Ellen G White claims that for at least the first 2 hundred years, Sunday worship was unheard of by Christians.

In the first centuries the true Sabbath had been kept by all Christians

Ellen G White's and Justin Martyr's account of Christian history clearly contradict each other. How do Seventh-day Adventists respond to Justin Martyr's claims?
While I don't believe that there is a standard response from the Adventist church on this.  One scholar has proposed that the appendix in which this statement was written, was a forgery, and added after the death of Martyr.

You can refer to the article here.

Adventist would argue that regardless of the contradiction that Sabbath is a biblical concept that has no scriptural evidence that it has been changed to Sunday.