Calendar does not show times in month view, and there is no box on Preferences to Show Times in Month View, as some have said there is.
if your speaking in reference to iCal then you can't. Just in Week and Day View. If you are using iCal and you have an event for the whole day there is no need to show the hours right, because you know it's an all day process. iCal determines that by putting those events at the top of the list in the day of the month. BUT If you have an event that is set by time, it will show in the right detail section the time your event starts, see picture below. However, if events overlap, say there is a coffee party before my 5k Run that runs until 9:30 there is no way to decipher that without clicking on the event to show the details of the event. Even though it will list Coffee Party before the 5k since it starts before you just won't know it overlaps. Everything is in chronological order. Your simply stuck with what Apple provides in this case.