The idea that food tastes better the next day is true for a specific set of dishes and false for the rest. What separates them is what happens chemically while the food sits.
Flavour compounds keep moving after cooking
Heat drives reactions quickly, but diffusion continues at room and refrigerator temperatures, and molecules keep migrating between ingredients long after the pot is off the hob.
Salt, fat-soluble aromatics and spice compounds move from the sauce into solid pieces and back, which evens out concentration differences that were still sharp at serving time.
Dishes with distinct components in a shared liquid gain most from this, which is why braises, stews, dals and curries are the classic examples.
Fat redistributes and carries aroma with it
Fat separates and rises during cooling, and as it does it carries dissolved aroma compounds with it, then re-emulsifies unevenly on reheating.
Reheated slowly and stirred, that fat disperses through the dish more thoroughly than it did originally, spreading aroma more evenly across every mouthful.
Skimming the risen fat before reheating removes some of that aroma, which is worth knowing before deciding to do it.
Starch retrogradation works both ways
Cooked starch granules begin to recrystallise as they cool, a process that firms the texture and, in some dishes, thickens the surrounding liquid usefully.
In a stew that firming is welcome, giving body to a sauce that seemed thin the night before without any reduction taking place.
In rice, pasta and bread the same process is the cause of staling, producing a dry, chalky texture that reheating only partly reverses.
Fresh, bright and crisp things decline
Volatile aromatics from fresh herbs, citrus and raw aromatics escape steadily and are largely gone by the following day.
Crisp textures depend on a dry surface, and any dish stored covered will equalise its moisture, softening anything fried or roasted.
Which is why dishes of this kind are better prepared as components and finished at the moment of eating rather than stored complete.
Building a week around the split
Cooking the improving dishes in quantity and the declining ones fresh makes better use of effort than treating all cooking as equally suitable for storage.
Bases, braised proteins, pulses and sauces hold and often improve, while leaves, fried items and starches are better made in small amounts.
Finishing elements kept separate, such as fresh herbs, acid and anything crisp, allow a stored dish to arrive at the table tasting made rather than kept.