The Greek government decided to make the VAT changes effective 20 July 2015 instead of 16 July 2015 which was the date of publication in the Gazette. The extra time was given to help businesses deal with the scope of the changes proposed in the legislation. The increase in the VAT rates in the Aegean Islands will start 1 October 2015 and continue for several months and the timetable will be finalized by the Ministries of Interior and Finance.
Here is an example of a note people received when the change was announced for the 16th: http://www.businessinsider.com/greek-hotels-hike-room-and-food-rates-overnight-forcing-tourists-to-pay-2015-7
However, the change will take effect Monday: http://www.reuters.com/article/2015/07/16/eurozone-greece-vat-idUSL5N0ZW4T520150716