Thursday is the 25th day, which is three weeks and four 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 that also marks 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.
Lag b'Omer, the 33rd day, falls next Thursday night, when customs of mourning are lifted and weddings are allowed. "Lag" represents the two Hebrew letters lamed and gimmel, whose numerical value is 33.