cal: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/cal - from package: util-linux-ng

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/cal - from package: bsdmainutils
/usr/lib/plan9/bin/cal - from package: 9base

You may install the required package with command apt-get install {package}

Error in other languages:
cal: bevel nie gevind nie
cal: comandă negăsită
cal: comando não encontrado
cal: commande introuvable
cal: command not found
cal: käsku ei ole
cal: không tìm thấy lệnh
cal: komanda nerasta
cal: Kommando nicht gefunden.
cal: kommandot finns inte
cal: komut yok
cal: nie znaleziono polecenia
cal: níor aimsíodh an t-ordú
cal: no se encontró la orden
cal: no s'ha trobat l'ordre
cal: opdracht niet gevonden
cal: parancs nem található
cal: perintah tidak ditemukan
cal: príkaz nenájdený
cal: příkaz nenalezen
cal: команда не найдена
cal: командата не е открита
cal: 命令找不到
cal: コマンドが見つかりません