Thursday is the 11th day, which is one week and four 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 blessing is preceded by the phrase, "I am ready to fulfill the mitzvah of counting the Omer." The Omer is mentioned shortly after the Almighty took the Jews out of Egypt and into the desert, where He supplied them with manna for food. Jews were commanded to gather an Omer's worth of manna every day, except on Friday when they gathered two portions, the second one being for the Sabbath. The Omer became a symbol of the precise amount needed for self-sustenance.