Checked in en-US and de-AT part of patch for bug 256087:

Input area is labelled Categories (should be Category)

calendar/resources/locale/de-AT/calendar.dtd

@@ -138,7 +138,7 @@
 <!ENTITY newtodo.startdate.label "Beginn">
 <!ENTITY newtodo.priority.label "Priorität">
 <!ENTITY newtodo.percentcomplete.label "&#037; fertig">
-<!ENTITY newtodo.categories.label "Kategorien">
+<!ENTITY newtodo.categories.label "Kategorie">
 <!ENTITY newtodo.starttime.warning "Ihr Beginn ist nach dem Ende.">
 <!ENTITY newtodo.cancelled.label "Abgebrochen">
 <!ENTITY newtodo.status.label "Status">

calendar/resources/locale/en-US/calendar.dtd

@@ -141,7 +141,7 @@
 <!ENTITY newtodo.startdate.label "Start Date">
 <!ENTITY newtodo.priority.label "Priority">
 <!ENTITY newtodo.percentcomplete.label "&#37; complete">
-<!ENTITY newtodo.categories.label "Categories">
+<!ENTITY newtodo.categories.label "Category">
 <!ENTITY newtodo.starttime.warning "Your start date occurs after your due date.">
 <!ENTITY newtodo.cancelled.label "Cancelled">
 <!ENTITY newtodo.status.label "Status">