CalendarAnything is a Salesforce “managed package” which does not permit tabs to be renamed the normal way. You can rename its tabs using Translation Workbench as follows:
If you are not already designated a translator:
Setup | Translation Workbench | Translation Settings
Make your language active, if not already; then
Edit | Identify Translators for this Language | [select user] | Save.
Now: Setup | Translation Workbench | Override.
Select package CalendarAnything, your language, and setup component Web Tab.
For Calendar, double click the empty cell under the column Custom Tab Label Override and set a new name. Save.