I've decided that a blogyear (or maybe blogjahr) is simply determined by a blog's very first post date and then ends on the previous day in the next calendar year. Therefore a blogyear is a calendar year, but doesn't necessarily begin on the 1st of Jan and end on the 31st of Dec like the Gregorian one does.
For instance Studio Mailbox was created on the 29th of February 2008. This means it's first blogyear was 29 Feb 08 - 28 Feb 09. It's second blogyear began on 1st March 09 and runs through 28 Feb 2010. And so on. Studio Mailbox is probably not the best example to use here because of the additional complication of it having started on a Leap Year.
So a blogyear is not to be confused with a fiscal or financial year, nor a budget year. It's certainly not a school year. It really has no connection to the Chinese zodiac year, although it might be nice to attach an astrological sign to your blog just for fun. Hmm. That makes Studio Mailbox a Pisces. Blub. Blub.