On October 01 starts the fourth and last quarter of the year 2017 which ends on December 31. A quarter is a fourth and in this case refers to the last three months of the year and always three sequenced months in general. This way the year is structured into four equal parts. October, November and December belong to the fourth quarter which is also called Q4 or in 2017 also Q4/2017.
So the 4th quarter spans a rather dark and uncomfortable season of the year on the Northern hemisphere. Whereas on the Southern hemisphere spring starts in the fourth quarter and the first summer days occur. Who wants to go into the warmth would travel the South in the fourth quarter. The structuring of quarters is especially important for the economic sector because data is usually summarized for the time period of one quarter. Taxes can be maturing at the end of a quarter. Many important days belong to the 4th quarter like the beginning of autumn and winter and holidays like Thanksgiving, Christmas and New Year's Eve.