calendar: 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/calendar - from package: calendar

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

On Debian possible default full paths are:

/usr/bin/calendar - from package: bsdmainutils

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

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