From Rabbi Samson Raphael Hirsch:

Mo’adim, appointed seasons, summon us to submit ourselves entirely to the contemplation and inner realization of those ideas which lie at their foundation. Just as Mo’ed in the spatial sense refers to the locality which men have as their appointed place of assembly for an appointed purpose, so Mo’ed in Time is a point in Time which summons us communally to an appointed activity—in this case, an inner activity. Thus Mo’adim are the days which stand out from the other days of the year. They summon us from our everyday life to halt and to dedicate all our spiritual activities to them. Front this point of view, Sabbath and Yom Kippur are also Mo’adim.
 
The Mo’adim interrupt the ordinary activities of our life and give us spirit, power, and consecration for the future by revivifying those ideas upon which our whole life is based.
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Rabbi Samson Raphael Hirsch zatz"l in Horeb