The green hairstreak (Callophrys rubi) is the only truly green British butterfly and so it is unmistakable, provided you see it in the first place that is. Being green and leaf shaped they are extremely well camouflaged and they have a fast fluttery flight so they quite often go undetected. However, it is a really lovely butterfly and a joy to behold.
They are generally in flight from about the middle of May until the end of June. Many years go by without me seeing them at all but in 2011, for example, by the end of May I had already seen them at three different locations. Those were my first sightings in Dorset since I moved here in 2006.
The green hairstreak is associated with the edges of woodland, scrub, heath and downland, anywhere there are plenty of shrubs about. Here in Dorset they should be seen in all sorts of places but, sadly, this does not seem to be the case. This might be a species that could benefit from better springs should the climate change that way.