I am to stupid for todos

Unfortunately I don't understand it and after 4 hours of searching I am now ready to ask for help.

The system is newly installed:
Version number: LuxCal Calendar: 5.3.2M
PHP 8.1.29, database: MySQL V10.6.18-MariaDB

I have created a category party and a category todo

What I want is to create an event that lasts one day e.g. birthday party 25.06 - so that the party can take place a lot has to happen.
Buy snacks 2 days before
1 day before room decoration
Heat up the barbecue 2 hours beforehand.

But I don't understand how I can now combine "shopping for snacks" on 23.06 with the event on 25.06 so that it appears in the ToDo list. This can be found at the top right with the hamburger menu.

I would be happy if someone could help before I delete the software in frustration