On Shabbat all sorts of cooking, changing matter, is forbidden. You can heat already cooked food but not boil water or liquefy solid fats. Jewish Holidays don't have these restrictions unless they fall on Shabbat. Shabbat overrides everything but Yom Kippur. And we have a Shabbat-Yom Kippur this year.
Back to topic, some people love cooking on chag, Jewish Holidays, because it means that they have less pre-Holiday preparations. Other people cook, because their families insist on "fresh" food. Decades, generations ago, prior to reliable electric refrigeration on had no choice. Food would spoil, so on Shabbat the cooked food stayed cooking until it was eaten; that's chulent. And on Holidays, especially during the two day Rosh Hashannah you had to cook. And a year like this one when Shabbat is immediately after Rosh Hashannah, there was no way to store cooked food safely. It's even permitted to slaughter animals for food on Jewish Holidays. And that's what people did, and to make things even easier, they cooked the meat or poultry on an open flame, so there was no need to do all the usual salting and soaking which takes hours.
I already have some chicken and meatloaf cooked and frozen for the three day Rosh Hashannah-Shabbat weekend. That means less work next week and more space in the refrigerator.
My plan is to cook all the food that needs to be cooked before Rosh Hashannah begins. All I'll have to do is cut the salad before each meal and make sure food is thawed out.
One of the reasons I prefer having everything cooked in advance is that I want to be able to go to synagogue and pray. I don't want to be a slave to the kitchen. For me having all the food cooked in advance frees me.