Indian festivals are each associated with specific sweets, and the associations are stable rather than arbitrary. The properties that qualify a sweet for that role are practical ones.
The ingredient has to be in season
Festival calendars follow agricultural cycles, and the sweets attached to them use what has just been harvested or what is abundant at that point in the year.
Sesame and jaggery sweets cluster around midwinter, when both are freshly available and when the warming quality attributed to them suits the weather.
New rice, fresh coconut and seasonal fruit anchor other festivals in the same way, which is why the same sweet rarely belongs to two festivals in different seasons.
It has to survive being given away
Festival sweets are distributed, which means they travel in boxes, sit at room temperature and are eaten over several days rather than immediately.
That rules out most soft dairy items and favors dense, dry, sugar-rich or fried sweets that hold their shape and resist spoiling.
The keeping requirement is why festival tables lean toward ladoos, dry barfis and fried items, while the perishable sweets stay in the shop.
It has to scale to a large batch
Households produce festival sweets in quantities far beyond a normal day's cooking, often over several sittings with several people working.
Sweets that qualify are ones where the process divides cleanly: cook a large mass, then shape it into many pieces, with the shaping done by whoever is available.
Anything requiring individual attention per piece during cooking does not scale that way, and those sweets remain shop products rather than home ones.
Religious constraints narrow the field further
Some observances exclude particular grains or ingredients, and the sweets permitted during those periods are built from what remains available.
Fasting-period sweets therefore use specific flours, nuts and dairy in place of ordinary grain, and those recipes exist almost entirely for that context.
The constraint has produced a whole parallel repertoire that would not otherwise have been developed, which is a recurring pattern in the cuisine.
The associations persist even where the reasons have lapsed
Ingredients are now available year-round and sweets can be bought rather than made, so neither seasonality nor batch scale binds a household the way it did.
The pairings survive anyway, because the sweet is part of what marks the occasion and substituting it would change the observance rather than the menu.
That is worth knowing before adapting a festival recipe: the constraints in it are historical, but the choice of sweet is not up for negotiation.