English has two words for the same concept, “freedom” and “liberty,” the former coming from Old English and the other from French. Armenian has only one, ազատութիւն (azadootioon), which has Iranian origin. Indeed, the adjective “free” is ազատ (azad), and when you hear or say “Free Armenia,” with “free” used as an adjective, that is ազատ Հայաստան (azad Hayasdan).
As any speaker of English knows, there is another use for “free,” which entered the language in the sixteenth century from the notion of “free of cost,” that is, not requiring an expense. The sweet sound of being told that something is free of charge does not probably get lost on anyone.
Of course, this meaning of “free” has nothing to do with freedom or liberty. You do not say that you enjoy freedom of charge, do you? This should give a hint to avoid falling into the abyss of funny translations.
If we go to a lecture and we do not have to pay for attending it, the advertisement will probably say that it is “free of charge.” The Armenian translation for that is մուտքը ազատ է (moodkuh azad eh), which means “access is free.” As you see, we do not use a literal translation like վճարումէ ազատ է (vujaroomeh azad eh), which does not exist in real life.
The adverb “gratis,” another word for this meaning of “free,” is much less used indeed. However, its Armenian translation, the adverb and adjective ձրի (tzuri), is very common. We use it any time that we do not have to pay for something or we do something without expecting a payment. We also use it to say that we did something for no reason.
Other than that, if they ask you when you are available to go out for dinner, think twice before saying “I am free on Friday” as « Ուրբաթ գիշեր ձրի եմ » (Oorpat kisher tzuri em) . To the casual listener, you might be implying that you do not charge on that day.
Since you actually wanted to say that you do not have any other engagements on that day, then the real answer should be « Ուրբաթ գիշեր ազատ եմ » (Oorpat kisher azad em).
Otherwise, get ready for a Homeric laughter (you may want to read the Iliad or the Odyssey to find out why it is called “Homeric”).