Ignatius of Antioch
Tradition tells us he was a disciple of John the Apostle. And later became the bishop in Antioch. He died in 108 A.D.
Ignatius is often quoted as an early witness to Sunday (or Lord's Day) observance.
The supposed reference to Lord's Day observance from the Magnesian letter, as it is often translated, is here quoted from Walter Martin's book "The Truth About Seventh Day Adventist, p. 152. reads as follows:
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If, then, those who walk in the ancient practices attain to newness of hope, no longer observing the Sabbath , but fashioning their lives after the Lord's Day on which our life also arose in Him, that we may be found disciples of Jesus Chris, our only teacher.


The quote, as quoted by Walter Martin and others appears at first glance to establish the early Christian usage of "Lord's Day" as referring to Sunday and replacing God's Sabbath. Whether or not Ignatius actually referred to Sunday as the Lord's day is open to question.

Scholars acknowledge that there are ambiguities in understanding the original text. For one thing the word "day" seems to be a later insertion. The meaning with the modifier "Lord's" can be "life" not "day" at all.

Fashioning their lives after the Lord's life,
Living according to the Lord's life, in which our life also springs up.
Makes far more sense than "living after the Lord's Day".

Copies close to the time of Ignatius, are pretty much non existent and what is found is fragmented, by the time we find complete copies of Ignatius letter, copied centuries later, Sunday worship had already gained a foothold. By that time the Latin word "dominicam" was commonly used to mean "Lord's day" for the first day of the week. However, that in no way proves Ignatius had that thought in mind when he wrote (in Greek).

Let's look at this portion in a wider context of Ignatius Magnesian letter:
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VIII
Be not led astray by strange doctrines or by old fables which are profitless. For if we are living until now according to Judaism, we confess that we have not received grace. For the divine prophets lived according to Jesus Christ. Therefore they were also persecuted, being inspired by His grace, to convince the disobedient that there is one God, who manifested himself through Jesus Christ, his son, who is his Word proceeding from silence, who in all respects was well pleasing to Him that sent him.
IX
If then they who walked in ancient customs came to a new hope, no longer living for the Sabbath, but for the Lord's day on which also our life spring up through him and his death, - though some deny him. and by this mystery we receive faith, and for this reason also we suffer, that we may be found disciples of Jesus Christ our only teacher, if these things be so, how then shall we be able to live without him of whom even the prophets were disciples in the Spirit and to whom they looked forward as their teacher? And for this reason he whom they waited for in righteousness, when when he came raised them from the dead.


Is the contrast between between Judaizing and living according to the Lord's day, or
is the contrast between Judaizing and living according to Christ's life?

The latter reading makes sense, we know the Jews burdened the Sabbath with hundreds of legalistic rules, Christ didn't do away with the Sabbath, nor is there any record of Him changing the date, He freed the seventh day Sabbath from the bondage of those hundreds of rules and invites us to worship Him in spirit and in truth.
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If, then, those who walk in the ancient practices came to newness of hope, no longer observing the Sabbath in the manner of the Jews , but fashioning their lives after the Lord's
life, by which our life also arose in Him, that we may be found disciples of Jesus Christ, our only teacher.


Who are "those who walk in the ancient practices"?
Are they the " divine prophets who lived according to Jesus Christ." (mentioned in VIII)
They kept the Seventh-day Sabbath, did they not? But not in the manner of the Jews, for they had found the newness of hope and were living according to their hope in Jesus Christ.

The long recension of the letter reads as follows:
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Let us therefore no longer keep the Sabbath after the Jewish manner, and rejoice in days of idleness; for "he that does not work, let him not eat." For say the holy oracles, "In the sweat of thy face shalt thou eat thy bread" But let everyone of you keep the Sabbath after a spiritual manner, rejoicing in meditation on the law, not in relaxation of the body, admitting the workmanship of God, and not eating things prepared the day before, nor using lukewarm drinks, and walking within a prescribed space, nor finding delight in dancing and paudits which have no sense in them.

Observe the Sabbath in the matter of keeping it in the original sense of spiritual, rather than by traditions that the Pharisees have setup for the sabbath.

Even in today's world, the word "sabbatizing " doesn't automatically mean Seventh-day Sabbath keeping. That word is equivalent to the general idea of Judaizing, or setting up of rules, rather than a day of worship in spirit and truth.

In looking at these writings of early Christians we need to realize they are all translations and often hundreds of years rest between the writer and the translation. We need to be careful not to place upon them the meaning that the Sunday keeping church placed upon them hundreds of years later in their attempt to link their departure from God's commandments back to early Christians.