Wednesday is the 17th day, which is two weeks and three days of the counting of the Omer, the measure of barley that was harvested in ancient times at the beginning of the 49-day counting period and brought to the Holy Temple as a sacrifice. The end of the counting period coincides with the evening before Shavuot, the festival of the first fruits and the handing down of the Torah on Har (Mount) Sinai. The blessing over the counting of the Omer is recited at night and is omitted during the day if one forgot to count.
Most of the days between Passover and Shavuot are a period of partial mourning in memory of 24,000 students of Rabbi Akiva who died during the same period. Haircuts, and dancing are forbidden but are allowed Wednesday night and Thursday, Yom HaAtzmaut (Independence Day). Weddings are prohibited until the month of Sivan, except for Lag B'Omer, the 33rd day of the Omer and the day on which Rabbi Alkiva's students were spared from death.