Learn Finnish: Oletko täällä perjantaina?

The ending -na is added to say that something happens "on" a certain day of the week. The base word is perjantai (Friday), and adding -na turns it into "on Friday". This ending is called the essive case.

Why does the ending for Friday use -na instead of -nä?

It is because of vowel harmony. The word perjantai contains the back vowel a. Therefore, the ending must match it by using the back vowel version, which is -na.

How does oletko work to mean "Are you"?

The word olet simply means "you are". To ask a yes/no question in Finnish, you add the question particle -ko (or -kö) to the word you are asking about. Attaching it to the verb makes it "Are you?".

I sometimes confuse the words for "here". Why is it täällä in this sentence?

Täällä is used when someone or something is stationary at a location, meaning just "here". If they were leaving or coming, you would use directional words like täältä (from here) or tänne (to here). Since we are asking if someone will physically be at the location on Friday, täällä is the right choice.