(ג) ...ונוהגין לשטוח עשבים בשבועות בבית הכנסת והבתים, זכר לשמחת מתן תורה. ונוהגין בכמה מקומות לאכול מאכלי חלב ביום ראשון של שבועות. ונראה לי הטעם, שהוא כמו שני תבשילין שלוקחים בליל פסח, זכר לפסח וזכר לחגיגה, כן אוכלים מאכל חלב ואחר כך מאכל בשר, וצריכין להביא עמהם שני לחם על השלחן שהוא במקום המזבח, ויש בזה זכרון לשתי הלחם שהיו מקריבין ביום הבכורים.
And We have a custom on Shavuos to put out grasses (vegetation) in shul and in our houses, to remember the celebration of the giving of the Torah. And there is a custom in many places to eat dairy foods on the first day of Shavuos. And it seems to me that the reason (for this custom is) that it resembles the two cooked foods we take on the seder night representing the Paschal lamb and the Festival Sacrifice; so to (on Shavuos), we eat milk and afterwards meat and need two separate breads to be put on the table, which represents the altar, and through this we recall the Two Loaf Offering of Shavuos.
Part I.
First we will examine the source of the custom to spread flowers and greenery around the shul and houses. Before we delve into the halachic ramifications, we will examine the sources for the custom.
(ב) בארבעה פרקים העולם נדון, בפסח על התבואה, בעצרת על פרות האילן, בראש השנה כל באי העולם עוברין לפניו כבני מרון, שנאמר (תהלים לג) היוצר יחד לבם, המבין אל כל מעשיהם.ובחג נדונין על המים.
(2) At four times the world is judged: On Pesach, for the crops. On Shavuot, for the fruits of the tree. On Rosh Hashnah, all the world passes before Him like sheep, as it says, "He that fashioneth the hearts of them all, that considereth all their doings." (Psalms 33:15) And on Sukkot, they are judged for the water.
Magen Avraham, Shulchan Arukh, Orach Chayyim, 494:3, SS 5
נוהגין להעמיד אילנות בבה"כ ובבתים ונ"ל הטעם שיזכרו שבעצרת נידונין על פירות העילן ויתפללו עליהן.
We have a custom to put trees in the shuls and houses, and it seems to me the reason behind this custom is to remember that on Shavuos we are judged with regards to the fruits of the trees, and we will thus pray for them (for a year of successful produce).
זכר לשמחת מתן תורה - שהיו שם עשבים סביב הר סיני כדכתיב הצאן והבקר אל ירעו וגו'.
"In Remembrance of the Giving of the Torah": For there was vegetation there all around Mt. Sinai, as it is written: "The sheep and cattle should not graze..."
...והגר"א בטל מנהג זה משום שעכשיו הוא חוק העמים להעמיד אילנות בחג שלהם.
... The Vilna Gaon abolished this minhag, because nowadays it has become the custom of other nations to erect trees on their holidays.
Now we will discuss the halachic ramifications of this custom.
We must first note that detaching any flowers or greenery from the ground or a tree or even a potted plant would be a clear Biblical violation of kotzeir, reaping, the third melacha of the 39 melachot (See Shulchan Aruch, Orach Chaim, Siman 336). The Shulchan Aruch (ibid.) also presents a gezeirah, a decree, against making any use of a tree on shabbos or Yom Tov lest one come to break something off of the tree. Those discussion points are of importance to our topic; however, often, the vegetation one would use for Shavuos foliage would often already be detached from the tree.
The issue of muktzah is much more practically relevant to our discussion.
There are a few aspects that must be examined with regard to vegetation:
A. Sticks, stones, and any other natural substances that were not designated for use or made into a vessel before shabbos/yom tov and are not useable for food or animal food are muktzah. (Shulchan Aruch, Orach Chaim, 308:20,28)
B. If one designates such an object for a specific use before shabbos, they would be permitted to move on shabbos/yom tov, meaning, such designation would remove them from the category of muktzah. (Shulchan Aruch, Orach Chaim, 308:20)
C. In addition, to understand the next source, one must realize that it it prohibited to prepare on shabbos for the needs of yom tov. (Shulchan Aruch, Orach Chaim, 494:3, Magen Avraham 5, and many other locations)
Magen Avraham, Shulchan Arukh, Orach Chayyim, 494:3, SS 5
If Shavuos falls on a Sunday, the Mahari"l holds one must spread the flowers/greenery before shabbos begins, as he holds that this greenery is not fitting to be used for animal food, and is therefore muktzah, as is brought in Shulchan Aruch, Orach Chaim, 308:28. It is possible to say, that being that this vegetation was designated before yom tov for a specific purpose on yom tov, they would not be muktzah, as is written in Shulchan Aruch, Orach Chaim, 308:20, and so is the custom to allow spreading them on yom tov itself. It is possible that even the Mahari"l himself only came to warn about spreading out the vegetation on shabbos, as that would constitute the prohibition of preparing for yom tov on shabbos.
See the Mishnah Berurah's conclusion to this halacha below
This is acceptable whether one spread the vegetation before yom tov or on yom tov itself. This is true regardless of whether they are fitting to be animal food, as one has designated them prior to yom tov, provided they are for smelling purposes. If Shavuos falls on a Sunday, one must spread the flowers/greenery before shabbos begins or on yom tov itself. Spreading out the vegetation on shabbos would constitute the prohibition of preparing for yom tov on shabbos.
The Mishnah Berurah adds that, in his opinion, although one could move the shavuos vegetation on yom tov itself it would need to have a specific yom tov related purpose other than the spreading, i.e. smelling it, otherwise, it should be spread prior to yom tov.
With regard to the halachos regarding aromatic vegetation, see the next source.
Chok Ya'akov, Shulchan Aruch 494 SS 7
The later commentaries write in the name of the Shela"h: "With regards to the custom to pass out vegetation to smell in shul, it should not be done until after the Shemoneh Esrei, in order for people to be able to make a bracha (borei minei b'samim), as it is forbidden to interrupt one's davening to make the bracha from the point of Baruch She'amar until after Shemoneh Esrei." In truth, the custom is to distribute smelling spices earlier in davening than that, as one can certainly interrupt for the bracha between paragraphs (bein ha'perakim) as we even allow one to interrupt to greet an honorable person or answer anyone between paragraphs.
Part II.
Now we will examine the source of the custom to eat dairy foods on Shavuos. Before we delve into the halachic ramifications, we will examine the sources for the custom. There are many sources and reasons behind this custom, I intend to focus on those offered by halachic commentaries.
The Rama explained in our first source that on the holiday of Shavuos, a two-loaf bread offering was brought in the Temple. Like on Pesach, we want to do something to remember the offering. In order to remember the offering, we create a scenario in which we will have to use two different breads (it is true that the custom is to always have two loaves for Shabbos or Yom Tov meals; however, one would not have to use both loaves, in the Shavuos scenario, one needs to use both loaves, as we will explain) thereby recalling the two bread offering. We accomplish this by having a dairy meal followed by a meat meal, which will require different loaves of bread, as we will explain.
(ד) מי שאכל גבינה ורוצה לאכול בשר צריך לבער מעל השלחן שיורי פת שאכל עם הגבינה ואסור לאכול גבינה על מפה שאכלו בה בשר (וכן להפך אסור) (כן משמע בארוך) וכל שכן שאסור לחתוך גבינה אפילו צוננת בסכין שרגילין לחתוך בשר ולא עוד אלא אפילו הפת שאוכלים עם הגבינה אסור לחתוך בסכין שחותכין בה בשר: הגה: וכן להפך נמי אסור מיהו על ידי נעיצה בקרקע קשה שרי (ב"י בשם א"ח וכל בו) אבל כבר נהגו כל ישראל להיות להם שני סכינים ולרשום אחד מהם שיהא לו היכר ונהגו לרשום של חלב ואין לשנות מנהג ישראל: