cal (Unix)

​cal​ is a standard program on Unix and Unix-like operating systems that prints an ASCII calendar of the given month or year. If the user does not specify any command-line options, cal will print a calendar of the current month.

$ cal

     April 2010
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

Thanks to : Wikipedia