How to find the definition of a string in /usr/include ?

[reg at dwf.com]

How do I find the definition of a string in /usr/include?

OK, there has got to be a better way to do this than with find and
grep (which isnt working).  BUT how does one find the definition
of a string in /usr/include ???

In particular, I KNOW that month is defined SOMEWHERE, but Im
not finding in in the obvious (to me) places. 

So how do I find 'Mo' or perhaps better yet 'Mo[' in /usr incude?

Is there a database that contains this information?
-- 
                                        Reg.Clemens
                                        reg at dwf.com