See the dates of the Hebrew feast days through 2012 listed below.

 * Pentecost (Shavuot) – Rabbinical tradition says “Shavuot” falls on the 6th of Sivan (50 days after Passover). In Leviticus 23:15-16, the bible clearly says it is the first day after the seventh Sabbath following Passover. This means it always falls on Sunday which may or may not be the 6th of Sivan. Hebrew tradition doesn’t line up with the bible in this case.

The Hebrew month is tied to an excellent measurement of the average time taken by the Moon to cycle from lunar conjunction to lunar conjunction. Twelve lunar months are about 354 days while the solar year is about 365 days so an extra lunar month is added every two or three years in accordance with a 19-year cycle of 235 lunar months.  A Hebrew leap year is one that has 13 months in it, a common year has 12 months.  Leap years of 13 months are the 3rd, 6th, 8th, 11th, 14th, 17th, and the 19th years.  In leap years, a 30 day month called Adar I is inserted immediately after the month of Shevat, and the regular 29 day month of Adar is called Adar II. This is done to ensure that the months of the Jewish calendar always fall in roughly the same seasons of the solar year, and in particular that Nisan is always in spring. Whether either Chesvan or Kislev both have 29 days, or both have 30 days, or one has 29 days and the other 30 days depends upon the number of days needed in each year.  Additionally, adjustments are needed to ensure certain holy days and festivals do or do not fall on certain days of the week in the coming year. For example, Yom Kippur, on which no work can be done, can never fall on Friday because the high fast could not be broken at sunset — because the end of Yom Kippur would be the start of the Sabbath, on which no work can be done. Thus some flexibility has been built in.

On average the calendar gets further out of step with the tropical year by roughly one day in 216 years. If the intention of the calendar is that Pesach should fall on the first full moon after the vernal equinox, this is still the case in most years. However, at present three times in 19 years Pesach is a month late by this criterion (as in 2005). Clearly, this problem will get worse over time and if the calendar is not amended, Pesach and the other festivals will progress through a complete cycle of seasons in about 79,000 years.  As the 19 year cycle (and indeed all aspects of the calendar) is part of codified Jewish law, it would only be possible to amend it if a Sanhedrin could be convened. This will only take place when the rebuilding of the Third Temple has begun, which will mark the salvation of the Hebrews according to Jewish belief.

A great site to use for Hebrew Calendars is www.abdicate.net/cal.aspx