"Plant garlic on the shortest day, harvest it on the longest" is one of the oldest pieces of New Zealand garden folklore, and like the Labour Weekend tomato rule, it holds up better than most tradition-based advice because there's a real mechanism behind it. Garlic needs a genuine cold spell, several weeks of low temperatures, to trigger bulb division, the process that turns a single planted clove into a proper multi-clove bulb rather than a small, round, single-clove "round" that's barely worth harvesting.
Planting around the winter solstice, 21 June, puts the crop through the coldest stretch of the year while it's still young, which is exactly when that cold exposure does the most good. Skip the cold period, or plant too late to get enough of it, and the result is often a garlic bed that looks fine all season and then disappoints at harvest with small, undivided bulbs.
The "harvest on the longest day" half of the rule is looser than the planting half, and treating it as a fixed date is a mistake in either direction. Garlic's actual signal to harvest is the foliage: once the lower third to half of the leaves have gone brown and dry while the top few stay green, the bulb is mature and ready to lift. In practice this lands anywhere from November to January depending on the season and how cold the winter was, with the December solstice as a reasonable middle estimate rather than a deadline.
The "harvest on the longest day" half of the rule is looser than the planting half, and treating it as a fixed date is a mistake in either direction.
Regional timing shifts the planting side more than most people realise. In Northland and Auckland, where winters are genuinely mild, a strict shortest-day planting can mean too little of the crop's early growth happens in real cold, and pushing planting slightly earlier, into April or May, gives it more of the cool season to work with before the ground properly warms. Further south, in Canterbury and Otago, the reverse applies: winters are cold enough that planting can wait until closer to the solstice itself without losing anything, since there's cold to spare either way.
The one mistake that undoes good timing entirely is planting garlic where alliums, onions, leeks, garlic itself, have grown in the last three to four years. White rot, a soil-borne fungal disease with no real cure once established, builds up in exactly this kind of repeated planting and can persist in the ground for years. Rotating the bed matters here more than it does for almost any other crop in the garden.