"If you were to come into a small town in Galicia during Shavuot, your eyes would be immediately drawn to numerous pictures cut out of paper looking at you from every window. And if you were to ask any Jewish boy what they are, he would look at you in surprise and say: "Are you a Jew? Don't you know what these are? They are 'roiselekh' which have always decorated the windows of our fathers and forefathers during this festival."
What do cut paper flowers have to do with Shavuot?
The tradition may have its origins in another hard to explain Shavuot minhag...
(ב) וֶהְיֵ֥ה נָכ֖וֹן לַבֹּ֑קֶר וְעָלִ֤יתָ בַבֹּ֙קֶר֙ אֶל־הַ֣ר סִינַ֔י וְנִצַּבְתָּ֥ לִ֛י שָׁ֖ם עַל־רֹ֥אשׁ הָהָֽר׃ (ג) וְאִישׁ֙ לֹֽא־יַעֲלֶ֣ה עִמָּ֔ךְ וְגַם־אִ֥ישׁ אַל־יֵרָ֖א בְּכָל־הָהָ֑ר גַּם־הַצֹּ֤אן וְהַבָּקָר֙ אַל־יִרְע֔וּ אֶל־מ֖וּל הָהָ֥ר הַהֽוּא׃
(2) Be ready by morning, and in the morning come up to Mount Sinai and present yourself there to Me, on the top of the mountain. (3) 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.”
אסור להתענות במוצאי חג השבועות:
הגה ואין אומרים תחנון מתחילת ראש חודש סיון עד ח' בו דהיינו אחר אסרו חג (הגהות מיימוני הלכות ברכות ומנהגים) ונוהגין לשטוח עשבים בשבועות בבית הכנסת והבתים זכר לשמחת מתן תורה ...
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 (Hagahot Maimoni, Hil' Tefillah Umnagim). 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...
{י} זכר לשמחת מתן תורה - שהיו שם עשבים סביב הר סיני כדכתיב הצאן והבקר אל ירעו וגו'. ...
נוהגין להעמיד אילנות בבהכ"נ ובבתים זכר שבעצרת נידונו על פירות האילן [מ"א]
והגר"א ביטל מנהג זה משום שעכשיו הוא חק העמים להעמיד אילנות בחג שלהם:
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..." ...
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.
The blue area represents Jewish Lithuania, the area where the Gr"a was most influential, The green area represents Galitzia, which is almost where all of the historical Shavuoslekh we know of come from.