According to Ezra, this tells us that the temple, which is a special place for God, was finished being built after a long time when King Darius was ruling. This was an important moment because it showed that God's people worked together to complete something important for worshiping Him.
Ezra 6:15: And this house was finished on the third day of the month Adar, which was in the sixth year of the reign of Darius the king.
Contradictory timeline for temple completion, as Haggai mentions another date.
Haggai 2:18: Consider now from this day and upward, from the four and twentieth day of the ninth [month, even] from the day that the foundation of the LORD'S temple was laid, consider [it].