When do you buy a book?

The social studies classes at school have just finished an economics unit.  Students had to research and create a product they thought would appeal to other seventh graders and it made me think about a book needing to be not only appealing for a child to read but also in the right place and time for parents to buy it.  When am I most inclined to purchase a new book for Maria or Estella?  Around a holiday- for a Christmas stocking, an Easter basket, a birthday present, as a gift for another child-again birthday but perhaps also christening.  Another time we often purchase books is when we're on vacation as souvenirs of our trip.  Any time we go to Spain we stock up on books of all kinds.  Some of our favorites are Luna and Cocodrilo by Antonio Rubio and Oscar Villan.  Years ago when we went to Rocky Mountain National Park on vacation I bought Antelope, Bison, Cougar: A National Park Wildlife Alphabet Book and in the gift store they stamped it with an official passport stamp of the national park.  A few years ago while spending a weekend on Monhegan Island with my family I bought The Country of Pointed Firs by Sarah Orne Jewett because it was about coastal Maine, my exact location at the time.  Anyone going to Maine with a middle schooler?  Think about buying them Touch Blue by Cynthia Lord, a great book about a small Maine island with a unique idea for keeping its one room school house open.  Where and when do you buy books?  What are some great purchases you've made when on vacation?  What makes a book as irresistible to you as a pink, animal print bracelet made out of duck tape is to a seventh grader?