What are the guidelines regarding Mass outside of a Church setting?

I was talking to a rather hardcore TLM'er last weekend who, when it was mentioned that we're having an all-city Mass next week held in an amphitheater outside of our courthouse (which I thought was cool) that only in emergencies - like a time of war - is it licit to have mass outdoors like that. I was like "we're at war with the culture" and he was like "mhmm", well anyway, the conversation didn't go so well. In any event, the previous weekend, I attended a TLM outdoors at camp and the week before I went to Mass several times in the Lucas Oil Stadium (Holy Ground now, next time Taylor Swift plays, I guess). In any event, there's gotta be some rubric around it. Is this just a "spirit of Vatican II" abuse that I've been frog-boiled into or is this actually something permitted by the US Bishops and is unique to the USA (but strangely ubiquitous) or is this a thing that can never be done under any circumstances?