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)?
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.
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".
'מועד' לשון רבים שניים מחכמי זמננו הסבירו שהמילה 'מועד' עצמה היא כבר בלשון רבים. הרב חיים קניבסקי כתב זאת בלשונו הקצרה ללא הבאת תימוכין לדברים, והרב חסידא תמך עצמו בפסוקים בצפניה (ג, יח) "נוגי מועד" ובאיכה (א, ד) "מבלי באי מועד"[כרמי ישראל, ירושלים תש"ן, עמ' ריט.]. גם אליעזר לוי במבוא לפירושו לסדר מועד מסכים שהתיבה 'מועד' היא לשון רבים, ומסביר: יתכן שביטוי 'מועדים' מכוון למועדים שבתורה בלבד, ואילו השם 'מועד' מקיף יותר וכולל כל הנכנס בגדר מועד קבוע, כגון חגים בתקנת חכמים ותעניות.
In a question and answer forum about three weeks ago this question was posed to R. Asher Weiss (approximately 4:20 into the recording). He said that the question never occurred to him but it is a good question. The Rosh Yeshiva of whichever yeshiva this took place in suggested that all the other sedarim deal with disparate things: there are various different types of zeraim, various types of kodshim, various types of nezikin, various types of nashim, various types of taharos. But when it comes down to it all holidays are really just one type of Moed. Thus, the other sedarim are plural while Moed is singular.
(I'm not sure I fully understand this answer.)
quite a few answer here but none which appeal to me. My own answer would be, that each yom tov is separate and none really have to do with each other. No yomim tovim are combined. Whereas zeroim different plants are talked about in each mesechta. The same with women in noshim. Each talks about the same person ie a woman. Nezikim is about a man doing some type of damage again the same man. Kodoshim, each one about korbonus animals, the same animals every time, ok menachos would be an exception. Taharos, is a bit harder to explain though.
The posuk calls shabbos a moed and rashi says its not really part of them. So since shabbos is included in the seder moed and not part of the moadim it prefers the singular moed and not moadim..
I remember seeing an answer which explains that the way the calander is set that will determine all the Yom tovim and it is like a chain not separate ,each are followed by the next.
Another answer similar to another given here is that shabbas is the ultimate moed and we give shabbas the name of moed.