The Mid-Autumn Festival falls on the fifteenth day of the eighth month in the Chinese lunar calendar, the night astronomers and folk tradition alike hold to bring the year's roundest and brightest full moon, and the mooncake takes its whole shape from that one night. The cake itself is built round, pressed in a wooden or plastic mold that stamps a pattern, often characters for longevity or harmony, into its top crust, and it is cut into wedges and shared rather than eaten whole by one person, so the division of a single round cake among a gathered family echoes the single moon shared by everyone who looks up at it that night.
Fillings vary regionally, lotus seed paste and red bean paste are the most traditional, and a filling built around a whole salted duck egg yolk is prized precisely because the yolk, sitting at the center of the pale filling, reproduces the full moon a second time inside the cake itself. Families gather outdoors after dark to look at the moon together while eating mooncakes and seasonal fruit, a practice historically called moon appreciation, and gifting boxes of mooncakes to relatives, neighbors and business contacts in the weeks before the festival is itself one of the year's major social customs, carrying the same theme of completeness and reunion the festival is built around.