Spread your wings and spot a butterfly
The organisation is holding the event in celebration of British butterflies at the time of the year when the greatest number of species are in flight.
Members of the public are being encouraged to tweet the names of species they spot this weekend and the postcode of where they spotted them to build up a digital butterfly map online.
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Hide AdBut the main focus of the weekend is to get outside, whether in cities or woodlands, and enjoy the "spirits of summer hours", according to conservation adviser Matthew Oates.
He said butterflies had long been one of the most popular elements of British flora and fauna, and butterfly collecting had once been a favourite pastime.
"What's happening now is butterfly collecting is coming back massively, not with nets and pins and killing jars, but with digital photography," he added.