words - Zeraim Moed Nashim Nezikim Kodshim Taharos - why not Moadim?

The names of the Shisha Sidrei Mishna (Six Orders of Mishnah) are: Zeraim, Moed, Nashim, Nezikim, Kodshim, Taharos. All of them besides Moed are in plural form. However Moed is in singular form. Why is it not called Seder Moadim (similar to the expression moadim l'simcha)?

Answer

From Soncino's intro to Seder Moed:

"It might be observed that the designation 'Mo'ed' is in the singular, as distinct from the plural forms used to designate the other Orders, e.g., Nashim, Nezikin, etc. It has been suggested that the singular is here specially used to avoid the confusion that might arise through the employment of the plural Seder Mo'adim (or Mo'adoth) denoting as it does in Rabbinic literature the Order of the Calendar¹. The opinion may, however, be hazarded that it is because the Sabbath and the festivals constitute one complete cycle of Jewish observance that preference has been given to the singular form."

^{1. V. Baneth, E., Die sechs Ordnungen der Mischna, Seder Mo'ed, p. 168.}