Learn Turkish: Bugün kimi ziyaret ediyorsun? - Whom are you visiting today?

Kimi is the accusative form of kim (who). We add the accusative suffix -i because the person being visited is the direct object of the verb. In English, this is exactly like the difference between who and whom.

I learned that to visit is ziyaret etmek. Why does it say ediyorsun with a d?

When the present continuous suffix -iyor is added to the helper verb etmek, the starting vowel i causes the letter t to soften into a d. This makes it much easier to say! So, et becomes ed, giving us ediyorsun.

Where is the word for you in this sentence?

In Turkish, the subject is built right into the verb. The personal suffix -sun at the end of ediyorsun tells us the subject is you (informal, singular). You could add the pronoun sen at the start for emphasis, but it is not necessary.

If this lesson is about ne zaman (when), why isn't it in this sentence?

This sentence already gives us the time using the word bugün (today). Because the time is known, the sentence asks about the object of the visit using kimi (whom) instead of asking when it happens.