Sunday is the 35th day, which is five weeks 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.
Following Lab b'Omer (the 33rd day of the counting of the Omer), many Israeli school children have adopted their own custom of Sfirat HaGomer" (counting until the end), which enumerates the number of days before the current school season ends on June 30.