I heard about this on the radio and thought it was a good excuse to sit in the garden for 15 minutes...
You just have to count the number of butterflies you see and log them on the above site. They even give you a really nice chart so you can butterfly spot as well as count!
You have to count them between today and the 1st of August.
Spent an hour today planting some flock and loads of leeks. I think they are leeks, might be onions! They got mixed up in the greenhouse. Mental note for next year, don't shun labels...