Board Thread:Questions and Answers/@comment-25083086-20140623183419/@comment-24203473-20140624130056

You'll know. The entire island will change appearance. For example: When an island is completely changed liked that, the related seasonal monster is available to breed. The Hoola, is available on two islands (the only seasonal monster to appear on more than one island). You can only breed the Hoola on Air and Earth islands while Earth Island is decorated with summer decorations. Hope this info helps!

PS - I would have included Water Island as the example picture or an empty Earth Island, but there aren't any empty pictures with them decorated up with their holiday decorations on the site.