Why are At and Holeches written with a Yud at the end that is not pronounced?
Radak: We find a superfluous Yud at the end, e.g. "Ohavti Ladush" (Hoshe'a 10:11). The pronunciation follows the majority 1 .
Why did he say that it is not [Rosh] Chodesh or Shabbos?
Radak citing Rosh Hashanah 16b: We infer that one must see his Rebbi on the festival 1 . It is not [Rosh] Chodesh or Shabbos - but if it were, she should go. Malbim - nowadays that there is no Mikdash, seeing one's Rebbi is in place of ascending for the festival, which was obligatory. Our verse teaches that seeing one's Rebbi is like receiving the Shechinah in the Mikdash! The custom was to go to Ish ha'Elokim on Rosh Chodesh and Shabbos. Hashem commanded to ascend for the Regel to learn Hashem's ways from the Kohanim, and so an influence of Kedushah will come on a person from the One who dwells in the chosen house. It would be proper to go also on Rosh Chodesh and Shabbos, like it will be in the future - "v'Hayah Midei Chodesh b'Chadsho u'Midei Shabbos b'Shabato Yavo Chol Basar Lehishtachavos" (Yeshayah 66:23). In this world it is not possible 2 , so they went on Rosh Chodesh and Shabbos to the Navi, on whom Hashem dwells, a semblance of how He dwells in the Mikdash. From him they learned Hashem's ways, and received an influence of Kedushah and Brachah. On the festival they went to the great Mikdash.
Radak citing his father: A month, and even a week, has not passed since you saw him. Why will you go to him now?
He should have said 'it is not Shabbos or [Rosh] Chodesh' (the more frequent one first)!
Divrei Eliyahu (Rosh Hashanah 16b): R. Yochanan ben Nuri (Eduyos 2:10) says that Resha'im are judged in Gehinom from Pesach until Shavu'os, for it says "Midei Chodesh b'Chadsho u'Midei Shabbos b'Shabato" (Yeshayah 66:23). I.e. "Shabbos" refers to Yom Tov. If not, Midei Shabbos b'Shabato should have preceded Midei Chodesh b'Chadsho! Also here, Shabbos refers to Yom Tov. This also explains how Rosh Hashanah 16b learns from here that one must see his Rebbi on the festival.
