There's a common misconception that February 29th (better known as Leap Day) is Sadie Hawkins Day. Although that isn't the case, February 29th does hold significance for women thanks to an old Irish tradition called St. Bridget's Complaint, which granted women permission to propose marriage on that day.
Sadie Hawkins Day is rooted in the story of Sadie Hawkins, a character created by Al Capp in the comic strip Li'l Abner. Sadie Described as "the homeliest gal in the hills," Sadie was unable get a date; so her father, a prominent citizen in the town of Dogpatch, named a day after her to help Sadie get a man. On Sadie Hawkins Day, a footrace was held in Dogpatch so the women could pursue the town's eligible bachelors.
According to the Li'l Abner website, Sadie Hawkins Day is an unspecified date in November which Al Capp observed in his comic strip for four decades.