Which feast did the returned exiles celebrate in the seventh month, before the temple foundation was laid?

Answer: Feast of Tabernacles

After setting up the altar in the seventh month, the exiles celebrated the Feast of Tabernacles as written, with daily burnt offerings.

“They also celebrated the Feast of Tabernacles in accordance with what is written, and they offered burnt offerings daily based on the number prescribed for each day.”— Ezra 3:4 (BSB)