What does the verse teach about strengthening their hands?
Rashi: They should build and finish, and not fear commoners who slacken and confound you from building, like it says in Ezra (4:4).
Radak: When you will hear these consolations for the future, your hands will be strengthened now to do Hashem's Mitzvos and build His house.
Malbim: The fourth reason why Bayis Sheni was small in their eyes is that they saw that the land was cursed, and it did not return to its strength. Chagai already prophesized that from when they found the Bayis, the curse will cease. From this they will know that the time for building came. The curse until now was because they delayed building.
When were "ha'Yamim ha'Eleh"?
Rashi: In the fourth year of Daryavesh. Malbim - it was two years after Chagai's prophecy [that the curse will cease from when they found the Bayis].
What matters do they hear?
Rashi: The consolations that the Nevi'im prophesized for them on the day that they made the foundation for Beis Hashem, in the second year of Daryavesh, to begin the work. They were already tested and known to be true Nevi'im (refer to 8:10:1:1). Therefore, it is proper to rely on their words of consolation.
What happened on the day that they began to build?
Radak: Brachah began to come upon you.
