Summer Is Summer by Phillis and David Gershator, illustrated by Sophie Blackall.
I really enjoyed several things about this book.  First the illustrations capture summer for me.  They are filled not with hot colors, but with blue skies, bright green grass, flowers and friends.  Second, the rhythm to the text is wonderful.  The refrain of “Summer is summer is summer” is repeated and that phrasing is repeated using other nouns.  It creates a structure to the book that allows the reader to just drift along with it. 
This is a perfect book to share on the final day of preschool when children are just waiting for that first official day of their summer to begin.