Wednesday is the tenth day, which is one week and three days, of the counting of the Omer, the measure of barley that was harvested in ancient times at the end of the 49-day counting period. The blessing over the counting of the Omer is recited at night and is omitted during the day if one forgot to count.
The counting is intended to remind Jews of the link between Passover, which commemorates the Exodus and freedom of slavery, which was not complete without the giving of the Torah, which is celebrated on Shavuot, one day after the completion of the counting of the Omer.