Calendar Thumbnails

Does anyone know how to get a thumbnail to appear on the calendar on a day with 2 or more events. It seems that a thumbnail will only appear if there is only the one event on that day. For example I may have event A from 1 June- 29 June and Event B 15 June only. Well on 15 June there will be a copy box listing the event info, with no thumbnail (as Event A also is on that day).