Well, I don't know what your native language is, but when we think of the English counterpart of this sentence we see that it's not possible at add any prepositions:
The week ends tomorrow.
The same happens if we translate it to Spanish:
La semana termina mañana.
Or if we translate it to Portuguese:
A semana termina amanhã.
I think the same would happen in French:
La semaine se termine demain.