Organize calendars by grouping similar calendars together, as shown below with the "Kids" group. If several people use your computer, each person could have a separate group for all of their personal calendars.
From the File menu, choose "New Calendar Group." Name the new group in the Calendars list, then drag individual calendars into the group by dropping them on top of the new group name. You can also create a new calendar group by Shift-clicking the New Calendar button (shown on the left).