Deepavali in Purattasi?

[NOV]

Deepavali 2009 is on October 17th.

Usually Deepavali is celebrated in the month of aippasi, but this year it appears during the month of purattasi. What gives?

Also Krishna Jayanthi is usually celebrated in Aavani, but this year there were two celebrations one month apart - one in Aadi (also celebrated by ISKCON) and another in Aavani (as observed by TN Vaisnavites)

A simple explanation would be appreciated.