The Downton Abbey series finale aired in the UK in 2015. The sixth and final series ran on ITV from 20 September to 8 November 2015, with its concluding regular episode broadcast on 8 November.
That episode brought several long-running storylines toward their resolution. Lady Mary and Henry Talbot's marriage was established, Edith's relationship with Bertie Pelham moved toward a happy ending, and Carson's declining health forced changes in the running of the house. The finale also looked ahead to a new chapter for Downton as the year 1926 approached.
There is a small scheduling detail behind occasional confusion about this answer. Downton Abbey also had a Christmas special broadcast on 25 December 2015, and some episode guides treat that special as the final episode of series six. Whether one means the final regular episode or the Christmas special, the year remains 2015. The years 2014, 2016 and 2017 are therefore incorrect.