I reckon the 21" model will be towards the end of November... In an ideal world each product would get its own launch window to ensure repeated footfall within the Apple stores. The second tranche (cellular) of iPad minis and 4th gen iPads seem to be coming around 16-21 November, so if I were Apple I wouldn't be looking to muddy the waters by launching the iMac then.
Move another week further on and you are either on 23 November or into the week commending 26 November, which is course is the last possible week if Apple is to make good on its November commitment.
As for the 27" models, they could follow one or two weeks later, so w/c 3 or 10 December. Any later than 14 December and punters will have their minds focused on other things as Christmas approaches. And even an early December launch might see many business purchases deferred until the new year.