What the harvest moon actually is
The harvest moon is the full moon nearest the autumn equinox. Around that date, the moon's path makes a shallow angle with the evening horizon, so moonrise comes only about half an hour later each night instead of the usual fifty minutes or more.
The result: several evenings in a row where bright moonlight arrives just as the light fades. Before electric light, that was not poetry. It was overtime.
Why it looks so orange and so huge
A moon near the horizon is seen through a thick slice of atmosphere, which scatters the blue light and leaves the gold. The 'giant moon' is an illusion — your brain comparing the moon to trees and rooftops — but it is one of the finest illusions nature performs.
Both effects are strongest exactly when the harvest moon is, which is why it has a name and the other full moons mostly don't.
The folklore
The harvest moon marked the last push of the farming year — the corn in, the work done, the feast earned. Harvest suppers, corn dollies and the last sheaf's little ceremonies all belong to its season.
A full moon at an autumn event is still read as generous luck: abundance, completion, the year giving its best before the dark half begins.
Planning an autumn event around it
If your event falls near the harvest moon, plan an outside moment after dark. Guests stepping out of a lit barn or marquee into moonlight like that will remember it longer than the centrepieces.
Autumn weather is a negotiation — warm afternoons, fast-cooling evenings, mist in the low ground. Layers, blankets and a warm drink station turn the season's moodiness into atmosphere.
Petitions under a full moon
The old craft keeps its workings by the moon, and a full moon is the traditional moment for asking big. If your date sits under the harvest moon, say so in your petition — it is the kind of detail a weather witch stays up for.