Osechi ryori is packed into jubako, stacked lacquered boxes usually holding three to five tiers, each tier traditionally reserved for a particular category of dish, so the boxes as a whole present a full, varied New Year meal without a single dish needing to be cooked once the new year has actually begun. That practice of cooking everything in advance, before the year turns, is tied to the belief that the kitchen fire and the household's cooking should rest over the first days of January, both to give the household's women, who traditionally did the cooking, a genuine rest from kitchen work, and to avoid disturbing Toshigami, the new year kami believed to visit the home over those first days, with the noise, smoke and activity of ordinary cooking.
Many of the dishes are also preserved by drying, pickling or heavy use of sugar and vinegar specifically so they will keep for several days without refrigeration, which is itself part of why the whole meal can be prepared ahead of the new year and then simply set out and eaten cold or at room temperature as the days pass. The custom of preparing food this way for Toshigami's visit reaches back through Japan's Shinto calendar of seasonal observances, though the elaborate, boxed osechi eaten today has become as much an expression of culinary craft and regional specialty as of household devotion.