The wiki has a page named
Gekka Matsuri ~la festa sotto la luna~ for the THE IDOLM@STER RADIO song, but shouldn't it be
Gekkasai ~la festa sotto la luna~?
Although 月下祭 is a word not found in a dictionary, I am pretty sure that any Japanese speaker of sufficient proficiency would read it
gekkasai, and this reading is also the one which the
Nico Nico Daihyakka article on the song provides.
Was the song mentioned as its title being read
gekka matsuri in the THE IDOLM@STER RADIO or an interview of Asami Imai herself?