for since the days of Jeshua
It is not meant that there had not been some formal observance of the feast of tabernacles (cf) (2 Chronicles 8:13); (Ezekiel 3:4), but that the people had not dwelt in booths since Joshua's days.
Related Commentaries of Nehemiah 8:17
Nehemiah 8:17
17 And all the congregation of them that were come again out of the captivity made booths, and sat under the booths: for since the days of Jeshua the son of Nun unto that day had not the children of Israel done so. And there was very great gladness.