The counting of the Omer is a practice of introspection and spiritual growth that has its origins in a Biblical commandment that no one practices anymore.
That commandment is to count forty-nine days beginning from the day on which the Omer, about two quarts of barley, was offered in the Temple in Jerusalem, up until the day before an offering of wheat was brought to the Temple on Shavuot. The Counting of the Omer begins on the second day of Passover and ends the day before the holiday of Shavuot.
In fact, the commandment wasn’t simply about counting – one was supposed to bring an Omer of grain to the Temple each day. Today, there is no Temple and most of us aren’t growing any barley, but Jews continue to count the days, because of the deep inner value of this particularly mystical practice. The counting of each of the forty-nine days between the Jewish holidays of Passover and Shavuot have come to symbolize the passage of the Jews freed from slavery in Egypt to the time they came before Mt. Sinai to receive the revelation of the Torah.
Thus, the idea of counting each day represents spiritual preparation and anticipation for the giving of the Torah, which was given by God on Mount Sinai on the fiftieth day, Shavuot. [This 50th day is the origin of the period Christians know as Pentecost]
The period of Omer is considered to be a time of potential for inner growth - for a person to work on strengthening one's middot or good characteristics and to work through one’s shadow side by meditating on the sefirotic aspects represented each day for the 49 days of the counting.
In Kabbalah, each of the seven weeks of the Omer-counting is associated with one of the seven lower sefirot (#4-10): Chesed, Gevurah, Tipheret, Netzach, Hod, Yesod, and Malchut. Each day of each week is also associated with one of these same seven sefirot, creating forty-nine permutations.
The first day of the Omer is therefore associated with "chesed that is in chesed", the second day with "gevurah that is in chesed"; the first day of the second week is associated with "chesed that is in gevurah," the second day of the second week with "gevurah that is in gevurah," and so on. Symbolically, each of these 49 permutations represents an aspect of each person's character that can be improved or further developed.
Rabbi Rami Shapiro offers a free download of his meditations on each day online. Rabbi Simon Jacobson explains these 49 levels in his classic book, The Spiritual Guide to Counting the Omer.
Because the sefirot are at the heart of Tarot symbolism, I use them as part of my meditative practice during this time, relying on a number of books, including Tarot and the Tree of Life. I also use the understanding of the sefirot learned in my time studying with Professor Elliot R. Wolfson at NYU, and with Jason Shulman at the Society of Souls (where I learned much, am grateful, and would not recommend to others). And of course, I bring my experience in Vipassana meditation and Buddhist study to this very Jewish practice as well. I also read the Pirkei Avot, a chapter of which is traditionally read each week between Passover and Shavuot.
On the night of Shavuot, once one has purified oneself through the Omer meditations, one is ready to receive the Torah anew, to receive one’s own personal revelation, a glimpse of the heavens and one’s destiny. The custom on that night is to stay up throughout the night in synagogue, devoted to prayer and study. I always look forward to this night. And I live with the understanding that to have expectations or desire about that night is to subvert the teaching; that every day counts, and that there is the potential for revelation in every moment.