According to history, the modification probably came at the request of an Alexandrian astronomer named Sosigenes, who rightly believed the intermittent addition of extra time to the yearly calendar would better synchronize the seasons with Earth’s orbit around sun.

She Pops the Question. Women propose marriage.

In 5th century Ireland Brigid of Kildare struck a deal with St Patrick to allow women to propose to men every 4 years. This was supposed to balance the traditional roles of men and women in a similar way to how Leap Day balances the calendar.

Leap Day has been known as “Bachelors’ Day” for this reason.

a law dated back to year 1288 by Queen Margaret of Scotland, on this day a woman can propose any man she likes – and the would-be husband is bound to accept her proposal, he could only refuse if he was already engaged. Fines were levied if the proposal was refused by man; compensation ranged from a kiss to £1 to a silk gown.

According to English law, February 29th was ignored and had no legal status, they just leaped over it. Folks assumed that traditions would also have no status on that day.

In many European countries, especially in the upper classes of society, tradition dictates that any man who refuses a woman’s proposal on February 29 has to buy her 12 pairs of gloves which she can wear to hide the embarrassment of not having an engagement ring.

Bad luck

In Scotland, it’s unlucky to be born on Leap Day.
In Greece it’s unlucky for couples to marry during a Leap Year, especially on Leap Day.

Sources:
http://marriage.about.com/cs/holidays/a/leapyear.htm
http://www.timeanddate.com/date/leap-day-february-29.html
http://www.groundreport.com/World/Leap-Day-Superstitions-Rock-Gender-Roles/2856354
http://www.nowpublic.com/culture/2012-leap-year-leap-year-facts-leap-day-birthdays-superstition-2896635.html#ixzz1nmq3wHSO

Reference: Old Superstitions: Which Ones Do You Live By?