It is prohibited to fast immediately after the holiday of Shavuot.

Ramah: And we do not say Tachanun from the beginning of Rosh Chodesh Sivan until the 8th day, which is after the post-holiday day. We have the custom to spread out plants on Shavuot in the synagogue and in houses, as a memory for the happiness of the receiving of the Torah. We have a custom everywhere to eat dairy food on the first day of Shavuot...

Spread out plants: And if Shavuot starts at the end of Shabbos, spread out the plants before Shabbos in accordance with the Maharil...

It is customary to place trees in the the synagogue and in houses, and it seems to me that the reason is so people remember that judgement of fruit trees occurs on Shavuos, and that they should pray for them.

At four times of the year the world is judged: On Passover judgment is passed concerning grain; on Shavuot concerning fruits that grow on a tree on Rosh HaShana, all creatures pass before Him like sheep [benei maron], as it is stated: “He Who fashions their hearts alike, Who considers all their deeds” (Psalms 33:15); and on the festival of Sukkot they are judged concerning water, i.e., the rainfall of the coming year.

In remembrance of the joy of receiving the Torah: for there were grasses around Mount Sinai, as it says "The flocks and cattle shall not graze..." Latter sages wrote that in places where it is the custom to give out fragrant grasses in the synagogue, they should not give them out from Barukh SheAmar until after the Amida, so that one may bless over them (the blessing for fragrant vegetation)-- because during that period it is forbidden to interrupt (the prayer). There is a custom to stand trees in the synagogues and homes in remembrance that on Shavuot judgment takes place for the fruits of the tree. The Gr"a abolished this custom because now it is the ritual of the (non-Jewish) nations to put up trees on their holiday.

No one else shall come up with you, and no one else shall be seen anywhere on the mountain; neither shall the flocks and the herds graze at the foot of this mountain.”

The Gr"a abolished the custom of putting trees where people assembled, because now the other nations stand up trees on their holidays of "Zolania Shvente" [Zielone Świątki] or "Fingshten" [Pfingsten]

And it is written that it is the custom to give out fragrant grasses on Shavuot in the synagogue, and in the houses, as a reminder of the joy of the giving of the Torah... and according to the Magen Avraham there was a custom to put up trees. But previous generations eliminated both of these customs for reasons that the great ones of the generation knew.

There is a custom on Shavuot to scatter roses in the synagogue and throw them on the Sefer Torah when it comes out and is returned.

There is also a custom to put roses and other flowers in oil lamps and hang them around the synagogue and to distribute flowers to the young children during the reading of Megillat Ruth in the synagogue on the second day for the children to take home...