Your day view has many sections that can be hidden, such as the day note, targets, the matrix, the task suggestion button, and your completed tasks. These can be toggled in the right hover menu:
Hover on the right side of your screen with your mouse then click "Show Completed Section" to toggle the visibility of your completed tasks.
For more details about how completed tasks are displayed and organized in your lists, see Showing completed tasks: inline, in a section, or hidden. That article covers where completed tasks appear in different views and how to customize those settings.