Wednesday is the 38th day, which is five weeks and three 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.
Torah sages teach that both the days and the weeks should be counted, based on two different sources. Counting the day is based on the phrase in the Book of VaYikra (Leviticus) "until the day after the seventh rest day, you shall count fifty days," and another phrase in the Book of Devarim (Deuteronomy) states "count weeks."