Thumbcal is here:
http://thumbapps.com/web/pages/thumbcal/overview.php
Use VJBrisk to change the default calendar app:
1. Download
VJBrisk app XDA Dev post #7.
2. Rename VJBrisk to Calendar.exe and the shortcut to your PIM app to (eg. ThumbCal.lnk) to VJTarget.lnk.
3. Place these two files into \Windows folder.