Learn Spanish (Latin America): ¡Ve a la fiesta y que te diviertas!

It can mean both depending on the context! Here, ve is the informal command form of ir (to go). You are directly telling a friend to go to the party. If it weren't a command, ve would mean 'he/she sees' from the verb ver.

Why do we need que before te diviertas?

In Spanish, you use que followed by the subjunctive mood to express a wish or a hope for someone else. It acts as a shortcut for saying 'I hope that...' or 'May you...'. So, que te diviertas translates to 'have fun', but it literally means something like 'May you have fun!' or '[I hope] that you have fun!'.

Why does diviertas end in an 'a' instead of an 'e'?

Because it is in the subjunctive mood! The base verb is divertir (to entertain), which is an '-ir' verb. In the normal present tense, the tú form is diviertes. But because this is a wish starting with que, we must use the subjunctive form, which swaps the normal '-e' ending of '-ir' verbs for an '-a' ending, making it diviertas.

Why is there a te before diviertas?