We do not know exactly when this was. It could have been in the late stages of the reign of Cyrus or it could have been in the very early stages of the reign of Darius. The author of Ezra is not concerned about sharing the particular year in which this event takes place but he wants to focus on the month.
1 When the seventh month came, and the children of Israel were in the towns, the people gathered as one man to Jerusalem.