My son (8 yrs.) came back from school today and asked me;
2) Why does the Gemore refer to Shavuos as Atzeres, isn’t that the name in the Torah for the last day of Sukos?

A. The Pesikta (Pinchas) mentions that our sages used as name the Targum Unkelus translation for this Yom Tov.
Ramban (Vayikra 23: 36) explains that the name Atzeres was given to make it similar to Sukos. Namely, that Shavuos is the concluding Yom Tov of Pesach, just as Shemini Atzeres is to Sukos. Therefore, the intermediate days of the Sefira are seen as Chol Hamoed.
Kad Hakemach (Atzeres) opines that the Jewish Nation while in Har Sinai were “neetzarim” to receive the Torah, due to the presence of the Shechina.
The Torah Temima (Devarim 16: 49) explains that the Torah used other names for Shavuos (Chag Hakatzir, Chag Habikurim), in order to differentiate from the last day of Pesach that also carries the name Atzeres (Devarim 16: 8).
Additionally, it may also symbolize that the count of the Omer, has come to its end.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a