There were many feasts that God required God's people to enjoy. Passover, and the feast of Unleavened bread were celebrated and are still celebrated. Pentecost is also still celebrated. There is one right between the two that is not fully celebrated anymore. The wave sheaf offering was presented.

Exo 23:15 You shall keep the Feast of Unleavened Bread. As I commanded you, you shall eat unleavened bread for seven days at the appointed time in the month of Abib, for in it you came out of Egypt. None shall appear before me empty-handed.

Between Unleavened Bread and the Feast of Harvest (Pentecost) there was the firstfruits of the barley harvest.
This barley harvest was the first harvest of the year, from this date they would count out 50 days until Pentecost, marking the beginning of the wheat harvest. (this would start the counting of the omer)

And the LORD spoke to Moses, saying, "Speak to the people of Israel and say to them, When you come into the land that I give you and reap its harvest, you shall bring the sheaf of the firstfruits of your harvest to the priest, and he shall wave the sheaf before the LORD, so that you may be accepted. On the day after the Sabbath the priest shall wave it. And on the day when you wave the sheaf, you shall offer a male lamb a year old without blemish as a burnt offering to the LORD. And the grain offering with it shall be two tenths of an ephah of fine flour mixed with oil, a food offering to the LORD with a pleasing aroma, and the drink offering with it shall be of wine, a fourth of a hin. And you shall eat neither bread nor grain parched or fresh until this same day, until you have brought the offering of your God: it is a statute forever throughout your generations in all your dwellings. "You shall count seven full weeks from the day after the Sabbath, from the day that you brought the sheaf of the wave offering. You shall count fifty days to the day after the seventh Sabbath. Then you shall present a grain offering of new grain to the LORD.
(Leviticus 23:9-16 ESV)

Jesus was put to death on Passover (14th of Nisan), put in the grave on (15th) the Unleavened bread, rose from the dead on the firstfruits (sheave offering of barley)

--sites from Jewish tradition:
jewfaq.com:

"On the second day of Passover, in the days of the Temple, an omer of barley was cut down and brought to the Temple as an offering."

- Is this offering still done? Anyone know?

chabad.com:

"The Omer is counted every evening after nightfall, from the second night of Passover till the night before Shavuot."

notice that it is said from the second night of Passover (that night would have been the day after passover, and the last hours of the unleavened bread celebration)

http://judaism.about.com/od/omer/f/omer_countwhy.htm:

"The Torah itself does not give us a reason for why we should Count the Omer. Over the years, Rabbis have provided us with several different explanations for this Biblical commandment.

In the Bible, Shavuot was an agricultural festival. Some say we were commanded to count the time from the second day of Passover when we gave an offering of the omer (sheaf of wheat) until the time of the wheat harvest when we needed to bring an offering of the new grain. "

I thought that was interesting - could anyone fill me in if the Orthodox Jews still participate in presenting the omer barley offering? I know the counting the omer (49) days is still kept.