Who knows twenty-eight?
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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 years in the מחזור גדול, the long solar cycle, which is used for saying ברכת החמה
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. :)
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.
28 Iyyar is the day we got the Temple Mount back.