Las cartas no llegaron el miércoles, el jueves.

Las cartas no llegaron el miércoles, SINO el jueves.

Explanation:

SINO is an adversative conjunction that expresses opposition or confrontation. In the example provided, said adversative coordination is "exclusive" since one statement excludes the other and the conjunction is used as a link. For example, no lo hice yo, sino mi hermana (I didn't do it, but my sister).  

answerestan corriendo; i believe this is correct, plz check first good luck;