You won't. The writings of the ECFs that were retained are mostly post-Bar Kochba, when Rome cracked down hard on anything that looked Jewish. So the church changed the day of worship from Saturday to Sunday. Writings that advocated for a continued Saturday Sabbath were not retained. Indeed, Eusubius, writing circa 300 ad, listed sabbath keepers like the Nazoreans and Ebionites as heretics.
The church did not "change" the day of worship. Christians were already worshiping on Sunday in the NT. The only exception was when they went to a synagogue and worshipped with Jews.
Try this verse.
10 I was in the Spirit on the Lord’s day, and I heard behind me a loud voice like a trumpet.
Revelation 1:10 RSV
This is John, the author of Revelation, telling us that he was "in the Spirit," that is, worshiping on "the Lord's day." Not the traditional Jewish sabbath, but the Lord's day, the day that Jesus was resurrected.
In case you doubt my reading of this, take a look at John Gill's commentary.
I was in the Spirit on the Lord's day
Not on the Jewish sabbath, which was now abolished,
nor was that ever called the Lord's day, and had John meant that, he would have said on the sabbath day; much less the Jewish passover, but the first day of the week is designed; so the Ethiopic version renders it "on the first day"; and is so called just as the ordinance of the supper is called the Lord's supper, being instituted by the Lord, and the Lord's table, (
1 Corinthians 10:21 ) (
11:20 ) , and that because it was the day in which our Lord rose from the dead, (
Mark 16:9 ) ; and in which he appeared at different times to his disciples, (
John 20:19 John 20:26 ) , and which the primitive churches set apart for his worship and service, and on which they met together to hear the word, and attend on ordinances, (
Acts 20:7 ) (
1 Corinthians 16:1 ) ; and Justin Martyr
F26 tells us, who lived within about fifty years after this time, that on the day called (th tou) (hliou hmera) , "Sunday", (by the Greeks,) the Christians met together in one place, and read the Scriptures, and prayed together, and administered the ordinance of the supper; and this, he adds, was the first day in which God created the World, and our Saviour Jesus Christ rose from the dead; yea, Barnabas
F1, the companion of the Apostle Paul, calls this day the eighth day, in distinction from the seventh day sabbath of the Jews, and which he says is the beginning of another world; and therefore we keep the eighth day, adds he, joyfully, in which Jesus rose from the dead, and being manifested, ascended unto heaven: and this day was known by the ancients by the name of "the Lord's day"; as by Ignatius
F2, Irenaeus
F3, Tertullian
F4, Origen
F5, and others ...
Link
Revelation 1:10 - Commentary & Verse Meaning - Exposition of the Bible