Question
When is the seven-armed Menorah used?
Answer
The Menorah that G-d commanded Moses to construct for the Tabernacle had 7 arms. This was used in Temple as well. The menorah that we use to commemorate the Chanukah miracle has 8 arms to commemorate that upon winning back the temple from the Greeks, and relighting the Temple menorah, the oil lasted for 8 days.
Rabbi Aron Yehuda Schwab