Pretty straight forward question, but why are the Seders of the Mishnah in plural (zeraim, nashim, nezikin, kedoshim, tehorot) but Seder moed is not moadim?
On the website of the congregation of Bais Dov Yosef, it gives a bunch of possible answers. By: Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a, see there:
- It infers to the Yom Shekulo Tov or Olam Habba, which is but one day in the eyes of Hashem (ibid. Tamid 5b, Oraissa p.321)
- The word Moed is actually already plural (Horav Chaim Kanievsky Shlit”a quoted in Chalichos Chaim 66 p.170, Introduction to Mishna Mefureshes).
- Even better, it hints that in the future all yomim tovim will become one single extended moed. Giving echo to the words of the Choze of Lublin Zt’l, that in the days of Moshiach, Tisha Beav will become the first day of yom tov, Shiva Asar Betamuz the last, while all the intermediate days will be an extensive chol hamoed. (Tapuchei Chaim ibid.)
- It points to the yom tov of all yomim tovim, the Shabbos Shabboson or Yom Kippur (Bais Yaakov [Komarna] quoted in Pri Chaim p. 279). Similarly, the masseches of Yom Kippur is Yuma (or Yoma) “the day”.
- It is an allusion to the saying (Yalkut Mishlei 9, Yotzeir P. Zachor) that all yomim tovim will be annulled with the exception of Purim. (Tapuchei Chaim p. 18, Noeh Doresh p. 103).