HGP is the Holiday Grand Plan - as found on the Organising Christmas website. (Me and links do not get on well together sorry). It's basically a 14 week plan, (which actually takes 16 weeks) starting on 1st September, giving you a week to deep clean each room, and also having various tasks each week to prepare for Christmas. Which should mean that, by Christmas week, you have had some fun times, all is organised and you are not stressed but ready to enjoy the holidays and whatever it throws at you. 
(The reality for most people is that they start off well, get something done in each room but not a full deep clean of everything done, get ahead on some holiday organising so while not stressed to the gills, are still doing some running during Christmas week - but not quite as bad as Asda Mum!!).