Calendars can be customized just like any other Visio drawing by applying themes and variants. Calendars can also be customized to show weather conditions or even moon phases.
To add a shape such as weather icons, scroll down the list of shapes in the Shapes pane and drag the desired icon onto a date in the calendar. You can align multiple shapes using the dynamic grids that appear when you try to align shapes next to each other.
You can also change the look and feel of the calendar by selecting themes and variants from the Design tab in the Ribbon.