Learn Polish: Latem popłyniemy na północ. - In the summer we will sail north.

Both latem and w lecie are correct and mean "in the summer." Using the instrumental case of a season, like latem from lato, or zimą from zima, is a very common and natural way to talk about when something happens in Polish.

The verb popłyniemy means "we will sail." How is it different from będziemy płynąć?

Popłyniemy is a perfective verb. It focuses on the completed action or the whole trip, like saying "we will make a sailing trip." On the other hand, będziemy płynąć is imperfective and focuses on the ongoing process, meaning "we will be sailing."

Why do we use na in na północ instead of do to mean "to the north"?

In Polish, compass directions always take the preposition na to show movement towards them. So, "to the north" is always na północ, just like "to the south" is na południe.

Does the word północ have another meaning besides "north"?

Yes! Północ literally means "half-night" (pół + noc), so it is also the Polish word for "midnight." You can easily tell which meaning is intended from the context.