Learn Polish: Widać, że przespałeś cały weekend. - You can see that you slept through the whole weekend.

It turns the imperfective verb spać (to sleep) into the perfective verb przespać (to sleep through). In this context, the prefix prze- expresses that the action of sleeping lasted continuously throughout an entire specified period of time.

Why is the time expression cały weekend in the Accusative case here?

When you use a verb with the duration prefix prze- (like przespać), the time period you spent doing that action must be in the Accusative case. Since weekend is a masculine inanimate noun, its Accusative form looks exactly the same as its Nominative (dictionary) form.

The English translation says "You can see", but why does the Polish sentence use widać instead of możesz zobaczyć?

Widać is an impersonal word meaning "it is visible" or "one can see". Polish strongly prefers impersonal expressions for general observations like this. If you said możesz zobaczyć, it would sound awkwardly literal, like you are commenting on the listener's physical ability to use their eyes.

How would this sentence change if I were saying it to a woman?

You would need to change the verb to match her gender. Przespałeś has the masculine past tense ending -eś. When speaking to a woman, you would say Widać, że przespałaś cały weekend.