I think we all need to take a step back and look for a second.
There are 52 weeks in the year = 52 Tuesday's a year.
Christmas Day this year falls on Tuesday.
Apple will not release an iMac on Christmas Day.
We're down to 51, but I have OCD so we're going to say 50.
The weather in the UK today is double 10 degrees celcius, which is 50 fahrenheit.
Taking all the above into account, we can all agree that we will definitely see an iMac on one of 50 Tuesdays (going through to 2013) but if the weather is 50 fahrenheit then the iMac release could fall on a Friday. Or a Saturday.