Some of Europe’s most eye-catching wonders come and go. To avoid missing out, here’s our guide to when and where to see them Sign up to Simon Calder’s free travel email for expert advice and money-saving discounts Get Simon Calder’s Travel email Get Simon Calder’s Travel email Email * SIGN UP I would like to be emailed about offers, events and updates from The Independent. Read our Privacy notice They're the natural European spectacles that appear and disappear as the year unfolds. Time it