'First Day' in the Bible

In the second year of Darius the king, in the sixth month, in the first day of the month, came the word of the LORD by Haggai the prophet unto Zerubbabel the son of Shealtiel, governor of Judah, and to Joshua the son of Josedech, the high priest, saying,

On the twenty-first day of the seventh month (Oct 17, 520 b.c., the second year of Darius king of Persia), the word of the Lord came by the prophet Haggai, saying,