Nobody is cooking and expecting it to be Eid, it’s how Eid is marked, cooking usually begins the night the new moon is sighted ime.
Basically you spend a month fasting (which ironically tend to involve richer food than usual), and then on the last day of Ramadan, sundown marks the end of fasting for that day, so you break fast at dusk (around 7pm here).
Then after the dusk and darkness prayers it's usual to celebrate the start of Eid, assuming it has been proclaimed, by banging drums, etc.
The main feasting takes the next day starting with breakfast, and there are various foods which take long cooking and things like chicken, beef would have to have been purchased in advance.
Also though Eid al Adha is nominally the festival of slaughtering cows, there are a lot of cows killed for Eid ul Fitri as well. In fact people might go the rest of the year without any beef at all. The cow slaughter for Eid Al Adha tends to be donated to neighbours, the poor, etc. , whereas for Eid ul Fitri you would buy by the kilogram.
Anyway the food for Eid ul Fitri tends to go on for a few days so there's something there on the second day as well, but IME when you get to the second day it's often 'off', because people tend to leave it in the wok without refrigeration, which isn't ideal in 30C heat.
I'm not really sure about the logistics of it all, but imagine if you had a turkey which expired on Christmas day and you didn't have a fridge and then Christmas was put back a day.
Yes, that's inconvenient.