i am speaking of roman ruler pliny the younger , early Christian historian justin martyr, clement of alexandria , before 200 a d, all wrote that Christians met on sunday.
they called it the 8th day.
Pliny the Younger, does not say 'first day of the week', neither '8th day'.
Here is what Pliny the Younger, states:
"... They stated that the sum of their guilt or error amounted to this, that they
used to gather on a stated day before dawn and sing to Christ as if he were a god, and that they took an oath not to involve themselves in villainy, but rather to commit no theft, no fraud, no adultery; not to break faith, nor to deny money placed with them in trust. Once these things were done, it was their custom to part and return later to eat a meal together, innocently, although they stopped this after my edict, in which I, following your mandate, forbade all secret societies. ..." -
[Pliny The Younger; Epistulae, Volume X, Number 96 [English]] -
http://www.tyrannus.com/pliny_let.html
"... Adfirmabant autem hanc fuisse summam vel culpae suae vel erroris,
quod essent soliti stato die ante lucem convenire carmenque Christo quasi deo dicere secum invicem seque sacramento non in scelus aliquod obstringere, sed ne furta, ne latrocinia, ne adulteria committerent, ne fidem fallerent, ne depositum appellati abnegarent; quibus peractis, morem sibi discedendi fuisse rursusque coeundi ad capiendum cibum, promiscuum tamen et innoxium; quod ipsum facere desisse post edictum meum, quo secundum mandata tua hetaerias esse vetueram. ..."
[Pliny The Younger; Epistulae, Volume X, Number 96 [Latin]] -
http://www.tyrannus.com/pliny_let.html
The "day" is not specifically given, nor specifically enumerated, and definitely doesn't say 'first day of the week', or '8th day' in English or Latin.
In the book of Acts, the discples would gather on sabbath (13:14,27,42,44, 15:21, 16:13, 17:2, 18:4), hundreds of times.
Luke specifically records the word ”sabbath” (the 7th day) 27 times (actually 29 times, with the koine Greek of Luke 24:1; Act 20:7 considered); Luke 4:16,31, 6:1,2,5,6,7,9, 13:10,14(x2),15,16, 14:1,3,5, 23:54,56; Acts 1:12, 13:14,27,42,44, 15:21, 16:31, 17:2, (3 consecutive sabbaths) 18:4 (“every sabbath”; also vs 11, a “year and six months”, which is 52 weeks + 26 weeks, being 78 consecutive sabbaths met together on by both Jew and Gentile with the Apostle Paul and others, and in Acts 18:23, “he had spent some time there” (several sabbaths); in Act 19:8, “for the space of three months” (12 sabbaths); in Acts 19:10, “continued by the space of two years” (104 sabbaths), “in Acts 19:22, “for a season” (several sabbaths), in Acts 20:3, “three months” (12 sabbaths), and in Acts 20:18,31, “all seasons, ”a space of three years” (156 sabbaths)).
Hebrews 4 (vs 9) also states the same, as does Revelation 1:10. More on those as needful.
The day begins with the "evening" in scripture:
[A.] Gen 1:5 And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
[B.] Lev_23:32 It shall be unto you a sabbath of rest, and ye shall afflict your souls: in the ninth day of the month at even, from even unto even, shall ye celebrate your sabbath.
[C.] Mar_1:32 And at even, when the sun did set, they brought unto him all that were diseased, and them that were possessed with devils.
[D.] Psa 104:19 He appointed the moon for seasons: the sun knoweth his going down.
Psa 104:20 Thou makest darkness, and it is night: wherein all the beasts of the forest do creep forth.
Psa 104:21 The young lions roar after their prey, and seek their meat from God.
Psa 104:22 The sun ariseth, they gather themselves together, and lay them down in their dens.
Psa 104:23 Man goeth forth unto his work and to his labour until the evening.
Psa 104:24 O LORD, how manifold are thy works! in wisdom hast thou made them all: the earth is full of thy riches.
Thus the disciples would eat, pray, and follow that by sleeping through the night, and awake before sunrise, as Jesus would (Joshua 3:1 (type); Mark 1:35), gather just before sunrise on Sabbath "morning" to worship their LORD, as He did (His Father), as given in "example" (1 Pet. 2:21).
Jos 3:1 And Joshua rose early in the morning; and they removed from Shittim, and came to Jordan, he and all the children of Israel, and lodged there before they passed over.
Mar 1:35 And in the morning, rising up a great while before day, he [Jesus] went out, and departed into a solitary place, and there prayed.
1Pe_2:21 For even hereunto were ye called: because Christ also suffered for us, leaving us an example, that ye should follow his steps:
There is no just before sunrise weekly meeting of disciples in the NT on the first day of the week. Even in Acts 20, Paul was up all night long, after sabbath was over, even until "midnight" (Acts 20:7) with a specific church of Christians in Troas, to 'confim' the church (as in Acts 15:41) as he had for the others as he travelled towards Rome. After the specical event with Eutychus, Paul continued his farwell with them until sunrise ("break of day") and left, for a 19 mile or so trip by foot to Assos. There is no repeat of this one time farewell event. It was not a weekly event, and there is no indication in English or koine Greek that it was so. The only weekly event in Acts 20, is the sabbath (vs 7), which occurs every seventh day, which is why Paul waited "seven days" (Acts 20:6) so he could keep sabbath with them, and would leave the following morning.
Thus, Pliny's "stated day before sunrise" is the Sabbath, the seventh day of the week (the pattern of Genesis to Revelation, and especially Acts), not the first day of the week (as there is no weekly pattern of followers of God meeting weekly on this first day of the week).