Who knows twenty-eight?
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Please cite/link your sources, if possible. After about one business day, I will:
Upvote all interesting answers.
Accept the best answer.
Go on to the next number.
28 years in the מחזור גדול, the long solar cycle, which is used for saying ברכת החמה
28 are the "times" listed in Koheles (Ecclesiastes) 3:2-8: "there is a time to be born and a time to die," etc.
A couple of Beis Hamikdash-related ones:*
28 amos (and four tefachim) was the length and width of the top of the Altar, in the second Beis Hamikdash (Rambam, Hil. Beis Habechirah 2:7), and it will be the same in the third Beis Hamikdash (may it be built soon!)
28 amos was also the length and width of the base of the Altar, in the first Beis Hamikdash. (Middos 3:1, R' Yose)
28 golden half-tubes (menakiyos) were used to hold up the 12 loaves of Lechem Hapanim ("showbread," or as they translate it in the Gutnick Chumash, "multi-surface bread") and allow air to circulate between them. (Rambam, ibid. 3:14)
28-ply threads (six plies each of blue wool, purple wool, crimson wool, and linen, and four plies of gold) were used to weave the Kohen Gadol's efod (apron) and choshen (breastplate). (Rambam, Hil. Klei Hamikdash 9:5)
* Knowing how fond Isaac is of those. :)
The letters in יהא שמה רבא מברך לעלם ולעלמי עלמיא. It's also the number of words from יהא שמה until דאמירן בעלמא. Both are gematria of כח (strength), so the Ashkenazi minhag is that the communal response is 28 letters long, and the Sephardi minhag is that the communal response is 28 words long. The 28 word restriction is why לעלא מן כל becomes לעלא לעלא מכל in the Ashkenazi minhag during the 10 days of teshuva.
28 is the length of each tapestry sewn together to cover the Mishkan, in Amot (cubits). This was enough to cover the top of the Mishkan and drape down over most of the two sides.
Rabbi Kaplan says something about this being the maximum size for a weavable tapestry. I think he means that they could make two 28 x 4s and sew them together, but not make a 28 x 8. Probably because 4 cubits is the average person's height (and therefore armspan), so it would be hard with a traditional loom to work with something wider.
Pirkei DeRabbi Eliezer 1:1 - the story about Rabbi Eliezer ben Hyrkanos and his desire to learn Torah.
28 Years is the age in which Rabbi Eliezer ben Hyrkanos was instructed by his father to get married.
ולמה את בוכה? אמר לו, שאני מבקש ללמד תורה. אמר לו והלא בן עשרים ושמונה שנים אתה, ואתה מבקש ללמד תורה. אלא קח לך אשה ותוליד לך בנים ואתה מוליכן לבית הספר.
BH
28 Nissan is the historic day that the Lubavitcher Rebbe told us that he has done everything he can, and now he's giving the mission over to us, to do everything we can to bring Moshiach, and that we get the "power" (gematria 28) to do so from the month of Nissan, the month of the redemption. (Said in the year 5751).
That same year on the 28th of Sivan the Rebbe publicized openly for the first time that the third beis hamikdash will descend from the heavens to 770 Eastern Parkway first, before flying to Yerushalayim with all of the synagogues etc.
וְהַיָּמִ֗ים אֲשֶׁ֨ר מָלַ֤ךְ יֵהוּא֙ עַל־יִשְׂרָאֵ֔ל עֶשְׂרִֽים־וּשְׁמֹנֶ֥ה שָׁנָ֖ה בְּשֹׁמְרֽוֹן
Jehu reigned over Israel for twenty-eight years in Samaria.
On the topic of reneging on a transaction - refer to Bava Metzia 49b
אלא דזבין שוי עשרים וארבעה בעשרין ותמניא תנן הורה רבי טרפון בלוד האונאה שמונה כסף מעשרים וארבעה כסף לסלע שליש למקח מאי לאו דזבין שוי שיתסרי בעשרים וארבעה וש"מ שליש מעות נמי שנינו
Rather, it is a case where he sold an item worth twenty-four ma’a for twenty-eight ma’a. We learned in the mishna that Rabbi Tarfon ruled in Lod: Exploitation is a measure of eight silver ma’a from the twenty-four silver ma’a of a sela, one-third of the transaction. What, is it not a case where he bought an item worth sixteen ma’a for twenty-four ma’a? And accordingly, one can conclude from the mishna that we learned that exploitation is also determined by one-third of the money paid, in accordance with the opinion of Shmuel. (Sefaria translation & notation)
Ezra 8:11 when listing the genealogy of the families that went up to Bavel in the reign of King Artachshasta:
וּמִבְּנֵ֣י בֵבַ֔י זְכַרְיָ֖ה בֶּן־בֵּבָ֑י וְעִמּ֕וֹ עֶשְׂרִ֥ים וּשְׁמֹנָ֖ה הַזְּכָרִֽים׃ {ס}
And of the sons of Bebai, Zechariah the son of Bebai; and with him twenty and eight males.
28 is the letter-count of the first pasuk in the Torah (Bereishis 1:1, see Kav HaYashar 80):
בְּרֵאשִׁ֖ית בָּרָ֣א אֱלֹהִ֑ים אֵ֥ת הַשָּׁמַ֖יִם וְאֵ֥ת הָאָֽרֶץ
28 is also the numerical value of the word for power, used in Tehillim 111:6 (koach).
corresponding to the twenty-eight letters in the first verse of the Torah, “In the beginning G-d created the Heaven and the earth” (Bereishis 1:1). This is an allusion to what is stated in Tehillim, “The power of His deeds He has declared to His people” (111:6). For the numerical value of the word “power” — koach — is twenty-eight.