Monday is the first day of the "counting of the Omer," the 49-day period which begins the second night of the Passover holiday and continues until the night before the festival of Shavuot (Pentecost). The omer is a measure of grain, which in Biblical times began to grow in the spring and was harvested after seven weeks, with the first produce being offered as a sacrifice at the Holy Temple. The Torah commands, "You shall count for yourselves, from the day after the rest day, from the day you bring the Omer of the waving; seven weeks, they shall be complete. Until the day after the seventh rest day, you shall count fifty days." The "rest day" has been interpreted as the first day of Passover, and counting 49 complete days ends on the eve of the 50th day, which is not counted.
Jewish law requires that a blessing for the appropriate day be recited at night, but if one forgets, he may count without a blessing during the following day, until nightfall, when the next day begins. Monday night is the second night of the counting.