A.T. Robertson Commentary John 20:26

"And after eight days again his disciples were within, and Thomas with them. Jesus cometh, the doors being shut, and stood in the midst, and said, Peace [be] unto you." — John 20:26 (ASV)

After eight days (μεθ' ημερας οκτω). That is the next Sunday evening, on the eighth day in reality just like "after three days" and "on the third day."

Within (εσω). Apparently in the same room as before.

Cometh (ερχετα). Vivid dramatic present. The other items precisely as in verse 19 save Thomas was with them.