Matthew Poole and continuators on John 20:26

26 ¶ And after eight days , again his disciples were within, and Thomas with them: Then came Jesus, the doors being shut, and stood in the midst, and said , Peace be unto you.

After eight days, signifieth here the eighth day from the Resurrection, counting the day wherein Christ rose for one; as we call those third days Agnes, which have but one days intermission, and those quartan Agnes which have but two days intermission; so Mark 8.31. It is said, after three days he shall rise, that is, the third day; This appears the most probable sense of the phrase; the Disciples beginning from Christs Resurrection to keep the first day of the week, for the weekly Sabbath; and having met on the Resurrection day, met again that day seven night, hoping (probably) for such a presence of Christ with them in their meeting, as they had before experienced; nor was their expectation vain. It appears also further, from Acts 20.7. and 1 Cor 16.2. That the Christians were wont ordinarily to meet together the first day of the week, for Religious exercises; which from Christs Resurrection, or Institution, or both, is thought to be called Rev. 1.10. the Lords day: Nor indeed do we find in all the Scripture of any Congregation of Christians, on the Iewish Sabbath, but upon this day; tho indeed, we find that the Apostles (and possibly some other Christians) did meet together with the Iews, in their Synagogues on their Sabbath; but we have not so much as one instance after the Resurrection, of any Congregation, where Christians onely were assembled upon the Iewish Sabbath. Thomas at this time was with them;

It is said again that Christ came, and stood in the midst of them, the doors being shut; concerning which phrase, see the notes on ver. 19.