We understand our leap years on our modern, Gregorian calendar. Every non-leap year has 365 days. Every year that is divisible by 4 is a leap year with 366 days. That extra day is added to the end of February to give it 29 days in a leap year. There's an additional rule for century years. Years divisible by 400 are a leap year with 366 days (such as the year 2000), but years not divisible by 400 are non-leap years with 365 days (such as the years 1900 and 2100).
To properly understand leap years in Bible history requires some knowledge of Hebrew culture, the Jewish calendar, and Israel’s geography as it was in Biblical days.
Let’s begin by looking at the Jewish calendar, which is tied to the phases of the moon. The month of Nisan is the first month on the Jewish calendar and begins in our March-April time frame. (As an aside, the Jewish New Year Day, Rosh Hashanah, is not in Nisan, but in the fall month of Tishri.) Since the Jewish calendar doesn’t have the same number of days as our Gregorian calendar, the first of Nisan “moves” on our calendar. (Easter “moves” on our calendar because it is also tied to the phases of the moon. Since Jesus’ resurrection was on the Sunday after Passover and Passover is normally on the first full moon of spring, Easter is set on the first Sunday after the first full moon after the vernal equinox. One year it may be in April, the next in March.)
God gave specific instructions about two important events that are to occur in Nisan – the Passover and the Feast of Unleavened Bread – that determine when Nisan begins. Leviticus 23:5 says, “In the fourteenth day of the first month at even is the LORD’s passover.” Exodus 23:15 says, “Thou shalt keep the feast of unleavened bread … in the time appointed of the month Abib.” Abib is a Hebrew word that refers to a certain stage of barley as it reaches ripeness, which occurs in the early spring.  The month of Abib is now called Nisan. So the Feast of Unleavened Bread and subsequently some portion of the month of Nisan must always be in the spring.
The day for them began at sunset – where for us it begins right after midnight. So the first day of the week for them began on our Saturday at sunset and ends Sunday at sunset. The Passover meal was celebrated on the evening of the 14th of Nisan and the daylight hours (what we would consider the next day) was the Preparation day for the Feast of Unleavened Bread.
With the exception of the Sabbath, the weekdays have no names. They are simply numbered:
yom rishon = “first day” = (Sunday)
yom sheni = “second day” = (Monday)
yom sh'lishi = “third day” = (Tuesday)
yom revi'i = “fourth day” = (Wednesday)
yom chamishi = “fifth day” = (Thursday)
yom shishi = “sixth day” = (Friday)
The week culminates on the seventh day, the Holy Shabbat or Sabbath. That’s why you don’t find the words “Sunday” or “Friday” in the Bible. 
The Jewish calendar is based upon a lunar year with each month beginning and ending with the new moon. The problem with a lunar calendar is it doesn’t line up with a solar calendar. The moon circles the earth approximately 12.4 times in every solar year, so a 12-month lunar calendar loses about 11 days every year while a 13-month lunar calendar gains about 19 days every year. That’s why Jewish dates “move” relative to our Gregorian calendar. On a 12-lunar-month calendar, the month of Nisan (which remember is supposed to occur in the spring) would occur 11 days earlier each year and would get earlier and earlier until it occurred in the winter. So to compensate for this “movement” when Nisan gets too close to winter, a second month of Adar is added to realign the lunar calendar with the solar year. 
The current form of the Jewish calendar is attributed to the Sanhedrin president Rabbi Hillel II in approximately 359 A.D. This calendar standardized the length of each month and the addition of months over the course of a 19-year cycle, so that the lunar calendar realigns with the solar year. An ordinary (non-leap) year has 353, 354, or 355 days. A leap year has 383, 384, or 385 days. An ordinary year has 12 months, while a leap year has 13 months. 
With a standardized calendar, mathematical algorithms based upon astronomical tables can determine the first of Nisan not only for the future, but for the past as well. The most important requirement for Nisan was and is to ensure that the Feast of Unleavened Bread happens in the spring.
However, in Jesus’ day, they were not familiar with the standardized calendar approved by Rabbi Hillel and they didn’t use mathematical algorithms or a strict adherence to the vernal equinox. They made decisions based upon observations. One observation was that two witnesses had to report to the Sanhedrin that they had seen the first crescent of the new moon after the month of Adar.  Another was that there had to be the prospect of ripe barley. (There are those in Israel today who are calling for a dismissal of the standardized calendar and bringing back the practice of determining the first of Nisan by the sighting of the new moon that immediately follows the ripening of barley in the land of Israel.)
In today’s world, the first day of spring in the northern hemisphere is based upon the vernal equinox. That is the moment in time around the 20th of March each year when the center of the sun is directly above the equator. So, even though the weather may cause us to say we had an early or a late spring, we don’t change our calendar. However, if we used a system like they did in Jesus’ day, on the 28th of February we would check the farms to see if the barley was close to ripening. If the barley were ripened at least to the state called abib, we would declare the next day the first of March. If the barley were not close to being abib, we would add a second month of February to our calendar!
Why a concern about ripe barley? It was because the Law required them to offer a sheaf of barley on the16th of Nisan (Leviticus 23). In Temple times, after sunset and immediately following the high Sabbath (the first day of the Feast of Unleavened Bread), three seahs (that is one ephah, about five gallons or 20 liters) of the new barley crop were reaped into three baskets. In the morning, it was winnowed and sifted, then parched over a fire, ground into flour, and sifted, yielding one omer (a tithe of an ephah, about one-half gallon or 2 liters) for the Omer offering. It was mixed with one log (about 1 ½ cups or 1/3 liter) of pure olive oil. 
For the Sanhedrin to declare rosh chodesh (first of the month) for Nisan in Jesus' time, they would have had to inspect the barley fields in the early part of March and gain confidence that there would be enough ripe barley available to be offered over 2 weeks later. We cannot determine today the condition of the barley crop the year Jesus was crucified, but it appears from the accounts of travelers during that era that barley became ripe in the plains of Jericho by the beginning of April.  So unless they had a mild winter, the sure probability of the barley crop was sufficiently advanced to reach abib at this point would be in doubt. Considering that meeting the requirements of Leviticus would be of more importance than rushing into declaring the month of Nisan too early, they would be more prone to declare a second month of Adar – thus delaying the month of Nisan for another cycle of the moon.
Is there evidence from the Bible to indicate that the year Jesus was crucified was a year with a late Passover date? Yes, there is.
On the day after His Triumphant Entry, Jesus cursed the fig tree. In the spring, no fig tree in Israel is bearing. The normal, full leaf and fruit-bearing season for fig trees was June. Even though this tree must have been faulty – to be in leaf without fruit – the fact that it had leaves indicates summer was nearing. Furthermore, during this last week and while on the Mount of Olives, Jesus taught, “Now learn a parable of the fig tree; When her branch is yet tender, and putteth forth leaves, ye know that summer is near.” (Mark 13:28)  Chances are Jesus was pointing to a fig tree that was just putting forth its leaves. Jesus would not have found a fig tree in leaf had the Passover in that year been in March.
Therefore, we can conclude from Biblical evidence that the year Christ was crucified was during a Jewish leap year.
 “Abib (Barley)”, The Karaite Korner, http://www.karaite-korner.org/abib.shtml
 “Introduction to the Jewish Calendar”, JewishGen, http://www.jewishgen.org/InfoFiles/m_calint.htm
 Tracey Rich, “Jewish Calendar”, Judaism 101, http://www.jewfaq.org/calendar.htm
 “The Jewish Calendar”, Calendars Through the Ages, http://www.webexhibits.org/calendars/calendar-jewish.html
 “Hebrew Calendar”, Wikipedia, http://en.wikipedia.org/wiki/Hebrew_calendar
 “Counting the Omer”, Messianic World Site, http://ahavta.org/PDFfiles/Counting%20the%20Omer.pdf
 Rev. Henry Browne, Ordo Saeclorum. A Treatise on the Chronology of the Holy Scriptures, pg. 466
 Olaf Hage, “The Crucifixion Of Jesus”, Hage Productions, 1998, http://petragrail.tripod.com/tree.html
 “The history of the fig in the holy land from ancient times to the present day”, Economic Botany, Springer New York Volume 19, Number 2 / April, 1965
 Grace Ministries English Study Bible, first edition, notes to Matthew 21:19, pg. 62