Daily shows you what notes you've taken on a given day.
Cards will be show up on a given date, depending on whether it's modifiable targeted date (or if a card doesn't have this, date created property) matches the date selected.
The calendar in the sidebar overview, gives you a quick overview of your month. Day objects have a "heatmap" to indicate how many cards you have on any given day. Drag-and-drop is also supported for cards + dates, so you can easily drag a card title onto a different date, setting the new "targeted date" property.
You can quickly change days by clicking on the current date in the top left > "Change Day", or using Ctrl / Cmd + D to type in a new date with your keyboard.