This button calls up the Mozilla Sunbird calendar from Firefox - to version 3.* - and Thunderbird - to version 3.*. This version is intended for Windows 32-bit operating systems, principally Windows XP.
It depends upon having the current version of the Mozilla Sunbird calendar downloaded from the distribution site and installed on your computer.
The button is added by the Right-Click "Customize .." toolbar option and dragging the button to wherever you want it in the toolbar.
Sunbird logo copyright of and used by kind permission of Mozilla Foundation.