To teach the laws and the meaning of the festival on the day of festival:
Moshe established that the Rabbi of each community expound to his community, on every holiday, with regards to the miracles pertaining to the holiday. Thus on Pesach the Exodus is discussed, while on Atzeres/Shavuos, the giving of the Torah is discussed, and on Chag/Sukkos the surrounding of the clouds of glory is discussed. In addition, it was instituted for the day of the festival that the Rabbis expound the laws which are applicable for that day, that which is allowed and that which is prohibited. This is despite the fact that they had already expounded on these laws thirty days prior. Thus on Pesach the laws of Pesach are expounded, and on Atzeres/Shavuos the laws of Atzeres [Shavuos] are expounded, and on Sukkos/chag the laws of the Chag [Sukkos] are expounded. Practically, today, being that all the laws are already written in books, there is no longer a custom to teach the laws of the holiday on the holiday, but rather to expound on the meaning and miracles of the holiday.
When is the Drasha given: This speech is given after eating the morning meal, [and it extends] until midday.
On the day of the holiday the Rabbi of each community expounds on the meaning and miracles of that holiday.
The laws of Pesach and Sukkos are to be learned 30 days prior to the festival and on the Shabbos prior to the festival. One is to expound on the miracles of the festivals on each holiday, including Shavuos.
 Why is expounding on the laws of Shavuos mentioned here and not earlier regarding the Shabbos that precedes the Holiday? Perhaps because the Takana of Moshe is still in place today, and thus cannot be removed, while the Takana by time of Temple is today only based on reason, and thus the sages can decide that if the reason does not apply, then it need not be done. This is another proof today that the Takana of thirty days is no longer the same standard. Vetzaruch Iyun.
 Lit. Agadah, see 529 in Kuntrus Acharon that Admur defines Agadah to mean the miracles.