Most planting advice works forward from today: what can I plant right now? A Christmas harvest works the opposite way. Pick the 25th of December as a fixed target, then count backward using each crop's actual days-to-harvest, and the question becomes which sowing windows still fit.
Fast crops are the safe bets no matter how late in spring you're reading this. Radishes, ready in 25 to 35 days, can be sown as late as the third week of November and still be pulled well before Christmas, with time to spare. Lettuce, spinach, silverbeet and dwarf beans all sit in the 40 to 65 day range, which means sowing anywhere from mid-October onward still lands comfortably inside the window.
Potatoes are the genuine Christmas crop, and it's worth being precise about why. An early or second-early variety planted at Labour Weekend, in the last week of October, is ready to dig as new potatoes right around Christmas Day, small, thin-skinned and waxy rather than fully mature, which is exactly what a Christmas new-potato harvest actually is. A maincrop variety planted the same week won't be ready until well into the new year.
Potatoes are the genuine Christmas crop, and it's worth being precise about why.
Tomatoes are worth a specific correction here, since they get lumped in with the Christmas tradition more often than the maths supports. At 70 to 85 days, a tomato planted at Labour Weekend is realistically giving its first ripe fruit in early-to-mid January, not on Christmas Day itself. A genuine Christmas tomato needs to have gone in considerably earlier, September in the warmer, largely frost-free parts of the North Island, which is exactly why Labour Weekend tomatoes are still worth planting on schedule: they're a January crop, and a very good one, just not the one on the Christmas table.
Sweetcorn is the one worth a specific warning: at 80 to 100 days, it needs to have gone in by roughly mid-September to be certain of a Christmas crop, which has usually already passed by the time this kind of guide gets read. If you're past that window, treat sweetcorn as a late-January crop instead of forcing a Christmas deadline that isn't realistic anymore.
The general rule for anything not listed here: check the specific crop's days-to-harvest, count back from the 25th, and if today's date is on the later side of that window, sow now rather than waiting for a "better" week that no longer exists. A Christmas harvest rewards the gardener who does the subtraction in October, not the one who plants on the day and hopes.