According to the Jewish calendar, this is Elul 1. According to the modern (Julian) calendar, this is August 29, 520 BC. This is three months after the grain and corn harvest. It is the time for the fruit tree harvest (grape, fig, and pomegranate). So their fields and orchards had been filled with activity. But the temple grounds were silent.
The first day of the month was a holy day of worship, a festival to the Lord. On it special offerings were made to the Lord (Numbers 28:11–15) and trumpets were blown (Numbers 10:10;Psalm 81:3), calling the Lord to help his people. Each month was to begin in commitment to the Lord.1
It was the time of the New Moon offering. So it is a day where the Jews could consider the quality of their harvest so far, and to look ahead to what the rest of the harvest would yield.2 It is on that day, at that time, with many people in Jerusalem for the festival and special offerings, that Haggai receives a message from the Lord for the people.
1 In the second year of Darius the king, in the sixth month, on the first day of the month, the word of the LORD came by the hand of Haggai the prophet to Zerubbabel the son of Shealtiel, governor of Judah, and to Joshua the son of Jehozadak, the high priest: