Phrases without a definite or indefinite article

Phrases without a definite or indefinite article

Hello all,

I've seen several French expressions/phrases where there is no definite or indefinite article, where there would be one in English. 

For example. C'est bon signe meaning that's A good sign, j'ai rencard avec lui meaning I have AN appointment with him.

Is there a general rule that explains this ?  

Thanks,

Jean