HaMaor Volume 46 Number 3 Page 26 says that since all the Yomim Tovim are going to be nullified besides Purim when Moshiach comes therefore it is called Moed in singular form as the only Mesechta remaining will be Megila.

Otzar Kol Minhagei Yishurin Siman 7 * note says that since the names of the Shisha Sidrei Mishna are based on the Pasuk והיה אמונת עתיך חסן ישועת חכמת ודעת and Moed is based on עתיך which is singular form therefore it is known as Moed in the singular form.

Halichos Chaim page 170 - Rabbi Chaim Kanievsky Shlita - contends that the word Moed is plural although there is no explanation given.

Rabbi Chasida in Karmei Yisrael page 219 says that Moed is plural similar to Tzefania 3:18 "נוגי מועד" and Eicha 1:4 "מבלי באי מועד".

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