Some tasks don't have a specific time but happen anytime during the day. Others aren't about a spefic action that needs to be done, but just remind you about a upcoming birthday.
For these tasks, so-called All-Day Tasks, Structured reserves a special place at the top of the timeline.
You can decide for yourself what kind of tasks you want to put there. Anything from "Empty Trash", "Leo's Birthday" or "New Year" goes.
Creating an all-day task
Creating an all-day task is really easy. Just tap the ➕ to create a normal task. Then tap the big center block in the time wheel. This will bring up the advanced time editing dialog. There you can select All-Day in the picker and tap the ❌ to dismiss.
Now you've set your task to appear in this top space. You can change the day, by tapping the All-Day on ... button or add a specific time again by tapping Add Time.
Completing an all-day task
All-Day tasks can also be completed (sometimes this obviously does not really make sense). To complete an all-day task, you can either just long-press it or tap it to bring up the detail dialog and then hit Complete.
Completed all-day tasks have their color filled in and their title crossed off.