To be eligible for S corporation status for the current tax year, a corporation must file the form by the fifteenth day of the third month of the corporation's tax year. (inc.com)
... On the fifteenth day of this seventh month is the Festival of Sukkot, seven days for the L - RD... on the eighth day, there shall be a holy convocation for you. (religion.blogs.cnn.com)
If it was the Passover, this would be the beginning of the fifteenth day of the month of Nisan; if it was the night before the Passover, it would be the beginning of the fourteenth. (religion-online.org)