Tonight the blessing is,
Baruch ata Ha-shem eloheinu melech ha-olam, asher kidshanu b'mitzvotav vitzivanu al sefirat ha-omer.Then you say
Blessed Are you God, Sovereign of the Universe, who has sanctified us with commandments and commanded us concerning the counting of the omer.
Hayom sh'losha usheloshim yom, sh'heim arba-ah shavuot v'chamishah yamim la-omerThere are many ways to view these fifty days. In the Torah it is a counting of an agricultural period, beginning at the period of harvest. For the rabbis the counting assured that we would not become so lost in our labor that we forget when we are to go to Jerusalem for the pilgrimage festival of Shavuot. And the rabbis also affirmed that each day recognizes one of the 49 gates of understanding that Moses received from God. The 50th is not known to humanity, and is beyond our capacity to comprehend.
Today is 33 days, which are four weeks and five days of the omer.
Because traditionally Shavuot is understood to be the day on which the Torah was given at Sinai, and it concludes the counting of the Omer, the period of counting is seen as an opportunity for the spiritual regeneration and reflection that would allow us to receive Torah. Each of the fifty days should include self-examination and a gradual process of striving to be the best and most pure person one can be.
The first 33 days recount the period during which Rabbi Akiva's 24, 000 students were, according to a tradition recorded in the Talmud (Yevamot 62b), stricken with plague by divine injunction. Their cruelty and insensitivity to one another, and their jealous suspicion of one another, discredited their work as students of one of the greatest Sages in Jewish history.
On the 33rd day of the counting of the Omer, the plague was lifted. This day is called Lag Ba'Omer, Lag being the pronunciation of the two Hebrew letters whose numeric equivalent is 33, lamed (30) and gimel (3). The period of counting is treated as period of solemnity and mourning and thus the mourning practice of not cutting one's hair is observed. This is why I currently look like Bigfoot, or at least like George Harrison on the cover of Let it Be. I didn't think to get a haircut before Passover.
But in recognition of Lag Ba-omer and the lifting of the plague, haircuts are permitted. So I am waiting for tomorrow morning (thus, 33 and a third) so I can clean up a bit.
Some resources for the counting of the omer:
Wikipedia's entry on Lag Ba'omer
A take on relating the Sefirot to the attributes to be cultivated in each of the days of the counting.
The counting of the Homer