Sukkah, Chapter 3, Mishnah 12. The mishnah opens: "Barishonah hayah lulav nital bamikdash shivah, uvamedinah yom echad" - while the Beis HaMikdash stood, the lulav and the other species were taken in the Beis HaMikdash all seven days of Sukkos, whereas outside the Mikdash they were taken on one day only.
The source for the distinction between the Mikdash and the rest of the country:
The Torah says: "And you shall take for yourselves on the first day the fruit of a beautiful tree, branches of palm trees, a bough of a thick tree and willows of the brook" - the obligation to take them is stated for the first day of Sukkos alone. With regard to the Mikdash, however, the Torah says: "And you shall rejoice before Hashem your God seven days," and the understanding of the verse is that rejoicing with the four species "before Hashem your God" refers to the Beis HaMikdash. There, then, it is Torah law that the lulav be taken all seven days.
The enactment of Rabban Yochanan ben Zakkai regarding the lulav:
The mishnah continues: "Mishecharav Beis HaMikdash, hiskin Rabban Yochanan ben Zakkai sheyehei lulav nital bamedinah shivah, zecher lamikdash" - outside the Beis HaMikdash as well the lulav is taken all seven days of the festival, as we do nowadays, as a remembrance of the mitzvah as it originally was in the Beis HaMikdash.
A further enactment: the day of the waving:
The mishnah closes with a law that is not our subject and is not even connected to the Beis HaMikdash, but it too is among the enactments of Rabban Yochanan ben Zakkai: "Vesheyehei yom hanef kulo asur" - the day of the waving of the omer, the sixteenth of Nissan, is forbidden for the entire day.
What is forbidden on it? It is the bringing of the omer that permits the new grain to be eaten. Before the omer is offered, one does not eat from the new crop, and only once the omer was offered on the sixteenth of Nissan - at that very moment - did the new grain become permitted.
However, as the Gemara explains, there is a verse indicating that the very onset of that day permits the grain. This understanding cannot refer to a time when the Beis HaMikdash stands, since then the permission depends on the offering of the omer; perforce the verse deals with a time when there is no Beis HaMikdash. It emerges that by Torah law, when there is no Mikdash and no offering to bring, the new grain is permitted already from the beginning of the sixteenth of Nissan.
Even so, Rabban Yochanan ben Zakkai decreed that the entire day be forbidden even when there is no Beis HaMikdash. The reason for the enactment: perhaps the Beis HaMikdash will be built on that very day, and people will not pay attention to this, but will assume that they may eat from the beginning of the day, without realizing that once the Mikdash is built they must wait for the bringing of the omer. For this reason it was established that the sixteenth of Nissan is always forbidden, for the entire day, in the eating of the new grain, until that day comes to an end.