The Rema in Hilchos Shavuos (Orach Chaim 493:3) writes: “The custom is to spread grass(es) on Shavuos in the shuls and in houses, as a remembrance to the joy of Mattan Torah”.
The question is, what is connection between our adornment of a shul for Shavuos and the actual day of Mattan Torah?
Answers:
1) The Levush (Orach Chaim 494:1) explains that at the time of Mattan Torah the pasuk specified (Shemos 19:8) that the cattle and sheep were prohibited to graze in the area. If there was a prohibition to graze, it must be that Har Sinai (which was in a desert) became surrounded by grass at that time. Therefore, as a Zecher l’Mattan Torah, we do the same.
2) The Maharil, which seemingly is the earliest mention of this minhag, states that the custom is to festoon the shuls specifically using fragrant grasses and flowers. Several seforim source this to the Gemara in Shabbos (88b) which elucidates the pasuk in Shir HaShirim (5:13): לחיו כערוגת הבושם ... שפתותיו שושנים - “Your cheeks are akin to bundles of spices... your lips like flowers”, to be referring to Mattan Torah. Every dibbur [utterance] that Hashem spoke filled the world with an ethereal fragrance. In commemoration, we spread aromatic flowers on Shavuos.
3) The Bnei Yisaschar cites a mashal [parable] which the Medrash Rabbah (Vayikra, Parshas Acharei Mos 23:3) uses to explain the pasuk which refers to Klal Yisroel as, כשושנה בין החוחים – “akin to a rose among thorns” (Shir HaShirim 2: 2):
There was once a king who had an orchard, containing figs, grapes, pomegranates, apples and all types of fruits. He left the orchard for one of his slaves to look after, and after a few days he came back to check on it, and he find that it was full of weeds and thorns. The king became very upset, however, amongst the thorns he found a sweet smelling rose and he picked it and smelt it and was pleasantly surprised. The king remarked, “because of this one rose, the entire orchard will be saved”. The Medrash continues, that the same thing is with Klal Yisroel. Hashem created the world and after 26 generations He came to inspect the world and saw it was full of thorns and was deeply saddened, however, when He saw Klal Yisroel who declared “na’aseh v’nishma, we will do and we will hear” and accepted the Torah He was appeased, and in their merit the world was saved. Just like the rose saved the kings orchard, similarly, Klal Yisroel saved the world.
The Bnei Yisaschar writes, that to symbolize this, on Shavuos we adorn our shuls with flowers
4) The Chida (Birkay Yosef, Orach Chaim 494:6) cites an early, albeit infamous, source that this minhag originates from Haman HaRasha. According to the Targum Sheini on Megillas Esther (3:8) during Haman’s diatribe to king Achashveirosh haranguing Klal Yisroel, he regaled him with all the ‘bizarre’ customs of the Jews, one of which was the spreading and gathering of apples and flowers on the roofs of their shuls on Shavuos. Although we generally do not learn halachah from Aggadah, the Noda B’Yehuda explains that we still can and do glean minhag Yisroel.
5) The Sefas Emes (on Megillas Rus, Lekutei Yehuda, Inyanei Shavuos), citing the Chiddushei HaRim offers an additional reason. Since we know Moshe Rabbeinu was born on the seventh of Adar and was hidden at home for three months, before he was hidden in the reeds (a type of grass) on the banks of the Nile River, this means that he was actually saved by Basya on Shavuos. As Moshe was the leader through whom we received the Torah, we commemorate this by spreading grasses on Zeman Mattan Torah. Some make that same cheshbon [calculation] in reverse, that if Moshe Rabbeinu was born on the seventh of Adar, then nine months prior he must have been conceived on Shavuos.
6) Rav Yitzchok Nissim Pala’gi (Yefeh L’Lev vol. 2, 494:7) adds that Rashi explains the pasuk in Tehillim (45:1): למנצח על שושנים - “A song on the roses” to be referring to talmiday chachomim. As such, on Shavuos, the Yom Tov of talmiday chachomim (see Pesachim 68b), we honor talmiday chachomim in this manner.
7) The She’aris Moshe (Mo’adim U’Zmanim, Shavuos d.h. ta’am eisev) writes simply that ‘eisev’, grass, stands for the three names of Shavuos - Atzeres, Shavuos, and Yom HaBikkurim.
8) Finally, R’ Matisyohu Deitch shlita explains, that many grasses are used for healing. By Mattan Torah, Klal Yisroel were healed from all their blemishes, and there was no longer a need for these grasses. Therefore, we spread them in the houses and shuls to show that there is no longer any need for them.
