Learn French: En été, nous allons à la mer. - In summer, we go to the sea.

In French, we use the preposition en with almost all seasons: en été (in summer), en automne (in autumn), and en hiver (in winter). The only exception is spring, which takes au (au printemps). You shouldn't use dans, as that usually implies being physically inside a space.

Do I need to translate the word 'the' if I want to say 'in the summer'?

No, even though in English we often say 'in the summer', French doesn't use an article (the equivalent of 'the') after en here. You just say en été.

Why is it à la mer and not au mer?

The word for sea (mer) is feminine, so it takes the feminine article la. To say 'to the', you use à la. If the destination were a masculine word, like the park (le parc), you would have to combine à and le into au (au parc).

Could I use on va instead of nous allons here?

Yes, absolutely! In spoken and informal French, people very often say on va instead of nous allons to mean 'we go'. So, En été, on va à la mer would sound very natural in everyday conversation.