Cogli l'attimo.
This literally translates to "Seize the moment," but it conveys the same meaning as "Seize the day."
Here are some other options:
* Approfitta del giorno: This literally translates to "Take advantage of the day" and conveys a similar meaning.
* Non perdere tempo: This means "Don't waste time."
* Vivi il presente: This means "Live in the present."
The best choice for you will depend on the context and the tone you want to convey.