message box in Tkinter

[Jah_Alarm]

I need to display a message box at the click of a button. I od the
following:

from Tkinter import *

def msg1():
    messagebox.showinfo(message='Have a good day')


Button(mainframe,text="About",command=msg1()).grid(column=360,row=36,sticky=W)

I get the error msg 'global name 'messagebox' is not defined'

When I try importing messagebox from Tkinter i get an error message
that this module doesn't exist.

thanks