Parshanut su Esodo 23:37

The Festival of Ingathering. In Devarim (16:13) it is called the Festival of Sukkos. This name is not used here because the commandment to dwell in sukkos was instituted by Hashem at a later point to commemorate the restoration of the Clouds of Glory, which had been taken away after the Sin of the Calf. This took place on the first day of Sukkos after Moshe came down with the Second Tablets.