The title is a play on Izayoi Sakuya's name and Japanese names for phases of the moon[1].
Nanokagetsu (七日月) is the 7th day of the lunar phase, i.e. the first quarter moon. Izayoi (十六夜) refers to the 16th day -- the first day of the waning moon after the full moon.
This might also have something to do with Sakuya being a child in the comic.
