New potatoes dug fresh on Christmas Day are one of the genuine seasonal pleasures of a New Zealand summer, and the timing behind it isn't luck, it's arithmetic. Potatoes take 90 to 120 days from planting to harvest. Count backward from the 25th of December and Labour Weekend, in late October, lands almost exactly in that window, close enough that "plant potatoes at Labour Weekend" has become as fixed a piece of garden tradition as the tomato version of the same rule.
Early and second-early varieties are the ones that actually make the Christmas deadline. A late-maincrop variety planted at Labour Weekend is still bulking up well into February, which makes it a fine crop but the wrong one for this particular goal. Ask for an early or second-early seed potato specifically if a Christmas harvest is the target, since "potato" alone at a garden centre in October can mean any of the three.
Regional frost timing matters here too, in a way the tradition glosses over. Potato foliage isn't as frost-tender as tomato or capsicum, established plants can shrug off a light touch of frost that would kill a tomato outright, but new shoots emerging in the first few weeks are more exposed. In Canterbury and Otago, where frost risk commonly runs into November and beyond, an exact Labour Weekend planting can still get caught, and mounding extra soil over emerging shoots on a forecast cold night is the standard insurance against it.
Regional frost timing matters here too, in a way the tradition glosses over.
Once planted, the only real job is mounding, drawing soil up around the stems every few weeks as they grow, which stops developing tubers from being exposed to sunlight and turning green. Green potatoes aren't worth eating: light exposure produces solanine, a compound that makes them bitter and mildly toxic in any real quantity. From an early or second-early variety planted at Labour Weekend and kept mounded, digging the first plant on Christmas morning to check for a decent-sized crop is one of the more reliable payoffs in a New Zealand vegetable garden.