There's what looks like a brown cicada on a tree in the pic and they start showing up in July(Northern Hemisphere),so definitely July but the grass might get to that color a bit earlier than that.
I think it is green like that in June too. My birthday is the 15th and I remember it being pretty green then. The summer equinox starts soon after too.
I was actually looking to see if there was a grass color by month chart and I couldn't find one. Not easily anyway, if there is one. But I would say that late May to June is when you would see the 'peak' healthiest dark green color for the grass. And it would slowly drop each month that passed.