Learn Finnish: Tapaan sinut maanantaina. - I am meeting you on Monday.

That -na ending is called the essive case. In Finnish, to say that something happens on a certain day of the week, you simply add -na (or -nä) to the day. So maanantai (Monday) becomes maanantaina (on Monday).

Why is the word for 'you' sinut instead of just sinä?

Sinut is the object form of sinä. You use it when a person is the complete target of an action. Since 'you' are the person being fully met, you must use this specific object form.

The dictionary form of 'to meet' is tavata. Where does the p in tapaan come from?

This is due to a spelling pattern called consonant gradation. For this specific type of verb, the basic dictionary form uses a 'weak' consonant (v), but it changes to a 'strong' consonant (p) when you actually conjugate it into forms like tapaan (I meet).

The English translation is 'I am meeting', referring to a future event. Why is the verb tapaan in the present tense?

Finnish does not have a separate future tense! You just use the present tense for future actions, and the context—like saying maanantaina (on Monday)—makes it totally clear that the meeting hasn't happened yet.