עזרא, פרק ג׳, פסוק ו׳

The return to Jerusalem brought a profound desire to renew the service of God, even before the physical structure of the Temple could be rebuilt. On the first day of the seventh month, which is the holiday of Rosh Hashanah [ביאור שטיינזלץ], the returning exiles began bringing burnt offerings to God. However, this sacrificial service did not immediately become a constant practice. It was only after the holiday of Sukkot that the offerings were brought continuously [מצודת דוד].

During this early period, the foundations of the Temple had not yet been rebuilt [מצודת דוד]. The absolute absence of these foundations dated back to the destruction of the First Temple, when the Edomites joined forces with the Chaldeans and completely leveled the sanctuary down to its very base [רס״ג]. Despite the lack of a standing Temple, the people were able to resume the offerings because they successfully prepared and built the altar itself, carefully placing it on its exact original site [רס״ג, ביאור שטיינזלץ]. Because they were deeply afraid of the surrounding nations, the people conducted this renewed sacrificial service on the altar while sheltered under temporary tents [רס״ג].