I won’t explain in detail why we need a leap year or that all calendars have them. I am sure you understand that this is how we compensate for the fact that our planet takes a tiny bit more than a year to circle the sun.
Many weird and lovely superstitions, traditions, and beliefs have grown up around this date.
The most common tradition is that that this is the day when a woman may propose to her man. Supposedly this comes from an old Irish legend that St. Brigid struck a deal with St. Patrick to allow women to propose to men – and not just the other way around – every four years. Modern mores and customs make this a moot point in many relationships.
In some places, Leap Day is called “Bachelors’ Day” for the same reason. In the Middle Ages, a man was expected to pay a penalty, such as a gown or money, if he refused a marriage proposal from a woman on Leap Day.
On the other hand, Greeks consider it unlucky for couples to marry at all during a leap year, and especially on Leap Day.
In Scotland, it used to be considered unlucky for someone to be born on Leap Day. In the US, people born on February 29 are all invited to join The Honor Society of Leap Year Day Babies. Some people born on February 29th celebrate their birthdays on February 28th in non-leap years, but many prefer March 1. The Henriksen family from Andenes, Norway currently holds the official Guinness Book record for the most number of children born in one family on leap day. Karin Henriksen gave birth to three children on February 29; her daughter Heidi in 1960 and her sons Olav and Leif-Martin in 1964 and 1968, respectively. Giachino Rossini, Dinah Shore, and Al Rosen were all Leap Day babies.
On the downside of ways to celebrate 29 February, in 1692 the first warrants in the Salem witchcraft trials in Massachusetts were issued on Leap Day. Some of the people involved in that incident were spawn of the devil, but I don't think they were the women accused.