diff --git a/calendar/templates/default/sidebox.xet b/calendar/templates/default/sidebox.xet
index 451614b3b2..2bf9330e46 100644
--- a/calendar/templates/default/sidebox.xet
+++ b/calendar/templates/default/sidebox.xet
@@ -18,7 +18,7 @@ Egroupware
             <et2-textbox  type="hidden" id="first"></et2-textbox>
             <et2-textbox  type="hidden" id="last"></et2-textbox>
             <hrule/>
-            <et2-select-cat  id="cat_id" placeholder="All categories" onchange="app.calendar.update_state({cat_id: widget.getValue()});" emptyLabel="All categories"></et2-select-cat>
+            <et2-select-cat id="cat_id" placeholder="All categories" onchange="app.calendar.update_state({cat_id: widget.getValue()});" emptyLabel="All categories" multiple="true"></et2-select-cat>
             <et2-select  id="status_filter" onchange="app.calendar.update_state({status_filter: widget.getValue()});" noLang="true"></et2-select>
             <hrule/>
             <et2-calendar-owner  id="owner" onchange="app.calendar.update_state({owner: widget.getValue()}); return false;" multiple="true" allowFreeEntries="false" autocompleteParams="{&quot;checkgrants&quot;: true}"/>