'First Day' in the Bible

From the first day of the seventh month they began to offer burnt offerings to the Lord, but the foundation of the temple of the Lord had not been laid.

Then the [former] exiles did so. Ezra the priest and men who were heads of fathers’ households were selected, according to their fathers’ households, each of them by name; and they sat down on the first day of the tenth month to investigate the matter.

And by the first day of the first month they finished investigating all the men married to foreign wives.