Show that at least 3 of any 25 days chosen must fall in the same month of the year?

(I think I might have to use Proof by Contradiction, b/c we haven't done counting yet)

I am not sure how to do this?. I am thinking this:

There is 12 months in a year, and at least 3 means x>3. So 3 * 12 = 36.

But I dont think its right though.