You will need Visualforce to do this. You cannot do it by somehow configuring the standard interface.
An example on how to achieve this (not only with Events, but with any object) is below.
Page
<apex:page standardController="Event" extensions="theExtension">
<apex:form>
<apex:inputField value="{!myEvent.Subject}" />
<!-- Add All the fields you want here, possibly adding formatting as well>
<apex:commandButton action="{!save}" />
</apex:form>
</apex:page>
Class
public class theExtension {
private final Event myEvent;
public theExtension(ApexPages.StandardControllers stdCtr) {
this.myEvent = (Event)stdCtr.getRecord();
}
public PageReference save() {
upsert myEvent;
return null;
}
}