In full calendars and event lists, it is the month + year selected.
It does not apply to weekly view calendars.
You have three (3) options. Let's walk through each of them.
To find this design item...
Step 1: After you log in, click on Calendars (brown box below).
The Your Calendars page will open.
Step 2: Choose the calendar you wish to modify (brown box below).
Step 3: Click on Design (brown box below).
Step 4: Scroll down and click on Navigation (brown box below).
Step 5: Scroll down until you find the header Selected date.
To modify this design item...
Option #1: Default
If you don't choose a color, the month and year will be grey.
Option #2: Inherit from website
Click on Inherit from website (brown box below). In this option, the color is inherited from your website or other embeddable component. In this case, the month and year are now black.
Option #3: Custom color picker
Click on Custom (brown box below) and then click on the circle (brown arrow below) to choose your color. Note that now the month and year are blue.
Click Save at the bottom of the column to save your changes.