birthday: 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 Debian possible default full paths are:

/usr/bin/birthday - from package: birthday

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

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