、。
During the Edo period, moon-viewing parties were very popular.
Literal
Edo-era [at-には], moon-viewing [genitive-の] banquets [subject-が] very popular were.
A cultural-historical sentence. 観月の宴 (かんげつのうたげ/えん) is 'moon-viewing banquet' — 観月 means 'viewing the moon' and 宴 is a literary word for 'feast/banquet.' 江戸時代には places the statement in a historical time frame. Moon-viewing (月見/観月) is a traditional Japanese autumn custom with roots in Heian-era court culture, where parties were held to appreciate the full moon, write poetry, and drink sake.