The Torah reading on the morning of the first day of Shabuot includes the Aseret Ha’dibberot (Ten Commandments). Is it proper to stand when the Ten Commandments are read? At the time when the Ten Commandments were proclaimed, all Beneh Yisrael stood reverently at the foot of Mount Sinai, and thus one deem it appropriate to reenact this event by standing in the synagogue during the reading of the account of the Revelation at Sinai.
The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) addresses this question in one of his responsa, and emphatically rules that one should not stand during this reading. He writes that if we treat one part of the Torah with greater respect than other parts, we give the impression that only that section originates from God, and the rest is less important because it was not given by God. We believe that each and every letter of the Torah, and even each crown that adorns certain letters, was given by God to Moshe at Mount Sinai, and we must ensure not to give the impression that Hashem gave us only the Ten Commandments. Therefore, one should not stand during the reading of the Ten Commandments.
One may stand during the reading of the Ten Commandments if one’s father, grandfather or Rabbi receives the Aliya of this reading. It is our custom to stand as a sign respect when one’s father, grandfather or Rabbi goes to the Torah for an Aliya, and to remain standing until the end of the Aliya. One may stand to give honor to these people if they are called for the Aliya of the Ten Commandments, even though generally it is improper to stand for this reading. In such a case, one should begin standing immediately when the father, grandfather or Rabbi begins walking toward the Torah, in order to make it clear that one stands for the person receiving the Aliya, and not to afford special respect to the Ten Commandments.
There is a widespread custom to decorate the synagogue with fragrant flowers, branches and other greenery for the holiday of Shabuot. The Midrash tells that Hashem filled Mount Sinai with fragrant spices and plants for the event of Matan Torah, and we therefore adorn the synagogue with flowers and the like to commemorate this event. We use flowers and branches from trees for this purpose because on Shabuot we are judged with regard to the quality of the fruits.
The Vilna Gaon (Rabbi Eliyahu of Vilna, 1720-1797) opposed this practice, claiming that it violates the prohibition against following the customs of the gentiles. Maran, however, in Bet Yosef, writes explicitly that this prohibition applies only to customs that are not based upon any rationale. Decorating the synagogue in honor of Shabuot has a sound rational basis, as we have seen, and thus does not transgress the prohibition of "U’be’hukotehem Lo Telechu" (Vayikra 18:3), which forbids imitating the irrational practices of the gentiles. Just as, for example, it is certainly permissible to wear a tie even though gentiles wear ties, because ties are worn for the purpose of appearing dignified, likewise, it is permissible to decorate the synagogue on Shabuot in commemoration of Matan Torah, when Mount Sinai was decorated with fragrant plants.
Therefore, it is certainly proper to observe this time-honored practice and decorate the synagogue with flowers and branches in honor of Shabuot.
Summary: One should not stand during the reading of the Ten Commandments, which is read on Shabuot. If one’s father, grandfather or Rabbi receives the Aliya which contains Ten Commandments, one should stand in their honor as they begin walking toward the Torah, so it is clear that one stands in their honor, and not to give special honor to the Ten Commandments. It is customary to decorate the synagogue with flowers, branches and other fragrant plants on Shavuot in commemoration of Matan Torah, when Mount Sinai was decorated with fragrant plants.