Although the modern calendar counts a year as 365 days, a complete revolution around the sun takes approximately 365 days and 6 hours. Every four years, as an extra 24 hours have accumulated, one extra day is added to keep the count coordinated with the sun's apparent position.

It is however slightly inaccurate to calculate an additional 6 hours each year as the time actually taken for the earth to complete a revolution around the sun is in fact 365 days, 5 hours and 49 minutes. To compensate for the 11 minutes difference, a century year that ends in two zeros is not a leap year unless it is also evenly divisible by 400. This means that 1600 and 2000 were leap years, as will be 2400 and 2800, but 1800 and 1900 were not, and neither will 2100 and 2200 be. To correct the remaining error (which amounts to one day every 3236 years) it has been proposed that years evenly divisible by 4,000 should not be leap years; but this rule has not been officially adopted.

The Gregorian calendar repeats itself every 400 years, which is exactly 20871 weeks including 97 leap days. Over this period February 29 falls 13 times on a Sunday, Tuesday or Thursday; 14 times on a Friday or Saturday; and 15 times on a Monday or Wednesday.

The concepts of the leap year and leap day are distinct from the leap second, which results from changes in the Earth's rotational speed.

The leap day was introduced as part of the Julian reform. The day following the Terminalia (February 23) was doubled, forming the so-called "bis sextum"—literally 'double sixth', since as dates were described in the Roman calendar February 23 was 'the sixth day before the Kalends of March'. The first day of the bis sextum (February 24) came to be regarded as the intercalated or "bissextile" day. February 29 came to be regarded as the leap day when the Roman system of numbering days was replaced by sequential numbering in the late Middle Ages.

An English law of 1256 decreed that in leap years, the leap day and the day before are to be reckoned as one day for the purpose of calculating when a full year has passed. Thus, in England and Wales a person born on February 29 legally reaches the age of 18 or 21 on February 28 of the relevant year. In the European Union, February 29 officially became the leap day only in 2000.

There is a tradition that women may make a proposal of marriage to men only in leap years, further restricted in some cases to only February 29. There is a tradition that in 1288 the Scottish parliament under Queen Margaret legislated that any woman could propose in Leap Year; few parliamentary records of that time exist, and none concern February 29.^[1] Another component of this tradition was that if the man rejects the proposal, he should soften the blow by providing a kiss, one pound currency, and a pair of gloves (some later sources say a silk gown). There were similar notions in France and Switzerland. Generally, these traditions are considered urban legends (see ^[2]).

In France, there is a humorous periodical called La Bougie du Sapeur (The Sapper's Candle) published every February 29 since 1980. The name is a reference to the sapper Camembert.