According to data collected on social media, the most popular break up day of the year is SOON.
The date is:
DECEMBER 11th
However, this date is a little too late in the holiday season in the court of public opinion because there is a socially acceptable date to cut-off point before the holidays. After December 6th, it is apparently considered not socially acceptable to break up with someone after this date without a gift. So either get them a little something or wait until after the holiday season.
Source: Mirror UK