August 1st, 2021
The first Sunday in August is a chance to promote the special sibling bond that comes from having a sister. Siblings typically are first friends, getting into (and out of) trouble together. While many other familial relationships are celebrated throughout the year, National Sister's Day gives everyone a chance to pay special tribute to sisters, whether they live near or far. It can also be a day to show appreciation for any sister-like relationship with friends, sorority sisters, etc.
Source: lilylisto.com/national-sister-day/
