modify "cal " command to display calenders of the specified months.
$ cal jan....aug
cal is a binary, and so you can't modify it that easily.
$ file $(which cal)
/usr/bin/cal: Mach-O 64-bit executable x86_64
What you need (it seems) is a script to output the result of cal for the months from January to August? If so, I would suggest you ask it in our Homework Forum.