probably because july 7, or Tanabata in the Gregorian calendar, is a central point in the story. just like how they corrupted haruhi.tv on the 18th of december back in '07 (did i get the year right?)
雨 (ame) trilogy
[070428] アメサラサ (amesarasa)
[090925] 夏ノ雨 (natsu no ame)
[120831] イモウトノカタチ (imouto no katachi)
Google Translated
http://translate.google.co.uk/translate?u=http%3A%2F%2Fwww.amazon.co.jp%2F%25E6%25B6%25BC%25E5%25AE%25AE%25E3%2583%258F%25E3%2583%25AB%25E3%2583%2592%25E3%2581%25AE%25E5%2588%2586%25E8%25A3%2582-%25E8%25A7%2592%25E5%25B7%259D%25E3%2582%25B9%25E3%2583%258B%25E3%2583%25BC%25E3%2582%25AB%25E3%2583%25BC%25E6%2596%2587%25E5%25BA%25AB-%25E8%25B0%25B7%25E5%25B7%259D-%25E6%25B5%2581%2Fdp%2F4044292094%2Fref%3Dsr_1_1%3Fie%3DUTF8%26s%3Dbooks%26qid%3D1246985620%26sr%3D8-1&sl=ja&tl=en&hl=en&ie=UTF-8
Wiki
http://en.wikipedia.org/wiki/List_of_Haruhi_Suzumiya_light_novels
It looks like it has the search inside logo...and it is Amazon. So is this official?