enum external declaration and use

Hi Folks,
I am trying to use enum to define a set of named constants with values so that they can be used by multiple C files in my project. For example. I use the following snipet to define Months in a headers file externals.h, and then If I then try to use it as follows in any of those files, it does not work.

 enum MONTHS {M1 = 'Jan', M2 = 'Feb', M3 = 'Mar'};
....
....
extern enum MONTHS; // should it be extern enum MONTHS months; ?
....
.....
SendByte (M1);//(This routine display M1 on my serial port).

I get error ---> (...error 230 MONTH unknown struct/union/enum tag)

What is the problem and how can I fix it ?
I would like to get a better understanding of this enum business, if possible..

Thanks.

Bhal Tulpule