'First Day' in the Bible

In the second year of Darius the king [of Persia], on the first day of the sixth month (Aug 29, 520 b.c.), the word of the Lord came by Haggai the prophet to Zerubbabel the son of Shealtiel, governor of Judah, and to Joshua the son of Jehozadak, 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,