It's a nice day here in the UK; blue sky, cool, but not cold. I was sitting outside earlier, facing the early morning sun, my cat on my lap, birds chirping around me, and I felt peaceful. I realized, "this is zazen."
Zazen can be whenever you want, and wherever you want. It's great when it's peaceful, and a relaxing environment certainly helps you get there. But it can also occur in less peaceful times, if you let it. For now, I'm good at zazen when things are calm and relaxing; I need to work on the rest.