Literal Translation:
* Quel jour sombre ! (This is the most literal translation)
More expressive options:
* Quelle journée morose ! (This emphasizes the feeling of gloom)
* Quelle journée triste ! (This emphasizes sadness)
* Quelle journée maussade ! (This emphasizes the dullness and lack of sunshine)
The best translation will depend on the context and the specific meaning you want to express.