What is the default calendar program?

I've noticed that Ubuntu doesn't come with a default calendar program... why not?

Go to >> System settings>>Details >>Default Application

For a calender in the sense of a list of days grouped by weeks, months and years,
there are cal and ncal (same man page);

At 2014-10-07:

$ cal
    October 2014      
Su Mo Tu We Th Fr Sa  
          1  2  3  4  
 5  6 [7] 8  9 10 11  
12 13 14 15 16 17 18  
19 20 21 22 23 24 25  
26 27 28 29 30 31

(The [7] is shown inverted.)
To see more months, Use -A n or -B n to show n month after or before, -y for the whole year, or -3 for the current month with one month before and after:

$ cal -3      
   September 2014         October 2014         November 2014      
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  
    1  2  3  4  5  6            1  2  3  4                     1  
 7  8  9 10 11 12 13   5  6 [7] 8  9 10 11   2  3  4  5  6  7  8  
14 15 16 17 18 19 20  12 13 14 15 16 17 18   9 10 11 12 13 14 15  
21 22 23 24 25 26 27  19 20 21 22 23 24 25  16 17 18 19 20 21 22  
28 29 30              26 27 28 29 30 31     23 24 25 26 27 28 29  
                                            30

Use ncal if you need the calendar week, the index of the week in the year; It has a different layout also:

$ ncal -w      
    October 2014      
Su     5 12 19 26   
Mo     6 13 20 27   
Tu    [7]14 21 28   
We  1  8 15 22 29   
Th  2  9 16 23 30   
Fr  3 10 17 24 31   
Sa  4 11 18 25      
   40 41 42 43 44

it doesn't? if there isn't one you can easily install any number of calender application,
This one for unity, http://news.softpedia.com/news/Introducing-Ubuntu-Calendar-Lens-for-Unity-243676.shtml
here is a list best of list, http://www.ekoob.com/best-calendar-applications-for-ubuntu-10427/

Thunderbird does have calendaring, but it is in a separate extension called Lightning, you can install it from the software centre or

sudo apt-get install xul-ext-lightning

maybe we should think about installing this by default.