The Pink Ribbon is the official trademark of the National Breast Cancer Foundation. Pink Ribbon Day is the signature day of the NBCF and is officially the fourth Monday in October. October is also internationally known as Breast Cancer Month.
However, as Pink for October states:
The hope is that you turn your site pink (in whatever way works for your site), go out to that World Wide Web thing and educate yourself about the multiple issues related to Breast Cancer, then take that newfound knowledge and tell someone else what you’ve learned.