Wednesday is the third day of the counting of the Omer, which concludes on the 49th day, the evening before the festival of Shavuot (Pentecost). The blessing over the counting of the omer is recited at night, but one who forgot to recite it may count the omer without a blessing during the following day.
An omer is a unit of measure of barley, which during the era of the Holy Temple was harvested and brought as an offering.