The Pesach observance (Passover) is the memorial of the day God saved Moses and Israel from Egypt and showed that the man/god of Egypt, the Pharaoh was impotent to God's power.
The day before Pesach is the Fast of the Firstborn, a minor fast for all firstborn males, commemorating the fact that the firstborn Jewish males in Egypt were not killed during the final plague.
the evening of the 14th of Nissan in the Hebrew calendar and the 15th is the first day of Passover
There are many different fasts in Judaism, but Passover, the observance of God's deliverance of all Israel, is not a fast.
http://www.jewfaq.org/holidaya.htm < tells the dates and has great information on it's observance
Now for my own words and some Torah excerpts to support. I hope you will go to read Exodus in entirety.
Jews are required by direct commandment in the Torah to keep the Passover observance for an eternal memorial. Passover ( Pesach in Hebrew) is the celebration of the deliverance of the eternal covenant nation Israel, the Hebrews, from the yolk of idolatry, superstition, and physical slavery in Egypt. It was a reaffirmation of their identity as a nation dedicated to God through Torah and our memorial reaffirms this for each generation. . The events we retell at Passover were demonstration to Egypt that their deities were false and their man/god Pharoah impotent to God's power.
It is a very important part of Judaism and many central aspects of our identity as an eternal covenant nation are embedded in it's observance. 12:12 I will pass through Egypt on that night, and I will kill every first-born in Egypt, man and beast. I will perform acts of judgment against all the gods of Egypt. I [alone] am God.
That is a very important passage there..NOTE..God will enact judgment against the false gods..plural now..of Egypt. That was the purpose of the Passover sacrifice.
12:13 The blood will be a sign for you on the houses where you are staying. I will see the blood and pass you by (pasach). There will not be any deadly plague among you when I strike Egypt.
12:27 You must answer, 'It is the Passover service to God. He passed over the houses of the Israelites in Egypt when He struck the Egyptians, sparing our homes.'
The people bent their heads and prostrated themselves.
The Passover sacrifice serves to demonstrate the people’s courage, as they take the Egyptians’ object of worship and slaughter and eat it in the service of the God of Israel. Torah is clear that the sacrifice and eating of the lamb at Pesach was the killing and eating of the false god of Egypt to show allegiance to the real God. The continuation of the Paschal lamb sacrifice is an eternal remembrance of that deliverance from idolatry and slavery. This is why no uncircumcised male or Gentile could partake of the Paschal lamb. If Christians wish to honor and observe the Passover for the meanings that are commanded in the Torah, you are welcome to attend any synagogue's community seder. Without the Temple we no longer make the sacrifice of the Paschal lamb and eat it so that command (no gentile may eat of it) is not broken. 12:42 There was a night of vigil for God, [preparing] to bring them out of Egypt. This night remains for the Israelites a vigil to God for all generations
12:43 God said to Moses and Aaron, 'This is the law of the Passover sacrifice:
'No outsider may eat it.
12:44 If a man buys a slave for cash and circumcises him, then [the slave] can eat it.
12:45 [But if a gentile is] a temporary resident or a hired hand, he may not eat [the Passover sacrifice].
12:46 'It must be eaten by a single group. Do not bring any of its meat out of the group. Do not break any of its bones.
12:47 'The entire community of Israel must keep [this ritual].
12:48 When a proselyte joins you and wants to offer the Passover sacrifice to God, every male [in his household] must be circumcised. He may then join in the observance, and be like a native-born [Israelite]. But no uncircumcised man may eat [the sacrifice].
12:49 The same law shall apply both for the native-born [Israelite] and for the proselyte who joins you.'
Now, we have a bone representing the lamb on the plate. I have had non-Jewish friends attend many seders at my home in the past and they find that learning the Torah meaning they've often overlooked is very powerful in its messages.
the link below is a great source to discover more about Yom Kippur, the major fast
http://www.jewfaq.org/holiday4.htm
and the other fasts of Judaism
http://www.jewfaq.org/holidaye.htm
EDIT: Dave is giving a VERY wrong answer on some of that information if you are looking for the Torah's version of events and commands for observance. The CHRISTIAN concept of the holiday of Pentecost ( meaning weeks and called Shavuot in Torah) and his dates are definitely not those according to the Torah commandments there so are not valid for the Jewish observance ( it is forbidden to alter the Torah even the tiniest )
Shavu'ot, the Festival of Weeks, is the second of the three major festivals with both historical and agricultural significance (the other two are Passover and Sukkot). Agriculturally, it commemorates the time when the first fruits were harvested and brought to the Temple, and is known as Hag ha-Bikkurim (the Festival of the First Fruits). Historically, it celebrates the giving of the Torah at Mount Sinai, and is also known as Hag Matan Torateinu (the Festival of the Giving of Our Torah).
The period from Passover to Shavu'ot is a time of great anticipation. We count each of the days from the second day of Passover to the day before Shavu'ot, 49 days or 7 full weeks, hence the name of the festival. See The Counting of the Omer. The counting reminds us of the important connection between Passover and Shavu'ot: Passover freed us physically from bondage, but the giving of the Torah on Shavu'ot redeemed us spiritually from our bondage to idolatry and immorality. Shavu'ot is also known as Pentecost, because it falls on the 50th day; however, Shavu'ot has no particular similarity to the Christian holiday of Pentecost, which occurs 50 days after their Spring holiday of EASTER..The Christian Pentecost is definitely NOT the same as the Torah's Shavot.
It is noteworthy that the holiday is called the time of the giving of the Torah, rather than the time of the receiving of the Torah. The sages point out that we are constantly in the process of receiving the Torah, that we receive it every day, but it was first given at this time. Thus it is the giving, not the receiving, that makes this holiday significant.
Shavu'ot is not tied to a particular calendar date, but to a counting from Passover. Because the length of the months used to be variable, determined by observation (see Jewish Calendar), and there are two new moons between Passover and Shavu'ot, Shavu'ot could occur on the 5th or 6th of Sivan. However, now that we have a mathematically determined calendar, and the months between Passover and Shavu'ot do not change length on the mathematical calendar, Shavu'ot is always on the 6th of Sivan (the 6th and 7th outside of Israel. See Extra Day of Holidays.)
*** In case Dave's answer giving the Christian holiday of observance rather than the Torah commanded observance of ..this year Shavuot is on Jewish Year 5769: sunset May 28, 2009 to nightfall May 30, 2009 ****
it is already OVER by May 31st. Counting from the Christian Easter Sunday, rather than from the day after the shabbat of Passover gives you the Christian holiday of Pentecost date, not the date of the commanded observance in Torah. The Shabbat referred to below is the Shabbat of Pesach..the first and last days have NO work..are Sabbaths.
23:11 He shall wave it in the motions prescribed for a wave offering to God, so that it will be acceptable for you. The priest shall make this wave offering on the day after the first day of the [Passover] holiday.
Vehenif et-ha'omer lifney Adonay lirtsonchem mimochorat haShabat yenifenu hakohen.
23:12 On the day you make the wave offering of the omer, you shall prepare an unblemished yearling sheep as a burnt offering to God.
Va'asitem beyom hanifchem et-ha'omer keves tamim ben-shnato le'olah l'Adonay.
The wave offering of the OMER was done on the second day of Pesach. The first day of Pesach is considered a Shabbat no matter what day it falls on. it says specifically there in Leviticus from the day when you bring the omer..that's the second day of Pesach and has been observed that way since before either First or Second Temple's stood where it was observed this way..and the date is Still reckoned in this manner.
You shall count for yourselves -- from the day after the Shabbat, from the day when you bring the Omer of the waving -- seven Shabbats, they shall be complete. Until the day after the seventh sabbath you shall count, fifty days... You shall convoke on this very day -- there shall be a holy convocation for yourselves -- you shall do no laborious work; it is an eternal decree in your dwelling places for your generations. -Leviticus 23:15-16, 21
23:15 You shall then count seven complete weeks after the day following the [Passover] holiday when you brought the omer as a wave offering,
Usfartem lachem mimochorat haShabat miyom havi'achem et-omer hatnufah sheva Shabatot tmimot tihyeynah.
23:16 until the day after the seventh week, when there will be [a total of] 50 days. [On that 50th day] you may present new grain as a meal offering to God.
Ad mimochorat haShabat hashvi'it tisperu chamishim yom vehikravtem minchah chadashah l'Adonay.
In addition..the Christian observance of Pentecost is a form of replacement theology..they observe their acceptance of the Christian belief in Jesus, his apostles beli