"Real Bulgarian" probably means they have a "Made in Bulgaria" stamp on their birth certificate. And Lactobacillus Bulgaricus in their blood. :)
But anyway - I don't think it's too soon. Especially if you mean Sunday - a couple of days' warning is enough usually. Also, I've learned that no matter how far ahead you plan something, it's quite difficult to pick a time that works for absolutely everyone. A lazy Sunday afternoon is as close to that as you can guess, I think. And if it's after your Bulgarian lesson, here's your first topic of conversation - what did you learn today? Though after saying that it's easy to imagine a scene where several Bulgarians with polite smiles and a calm mixture of curiosity and eagerness await the answer while a slightly blushing Carolyn wonders "What did I get myself into" :)
But I digress. This weekend is OK, let's see what the others will say. As for the place, "outdoor" sounds nice. I say let's meet up and if it's raining, we'll decide on the spot.