simple accounting question

Not asking for advice don't worry..

I just wondering if anyone could tell me why woolworths and some other companies use variable year ending report dates ie. Woolworths 2011 was year ended 26th June, 2010 was year ended 27th June.

I know that 30 June is obviously not required by law, and some other Aus companies use other months, but they all tend to be on the last day of those months, ie. 30 Sep, 31 Dec etc and don't change.

Why is woolworths not on end of month cycle, and why does the date change each year ? It must be a simple accounting answer ?