General fact: 28 is the number of days the curing time of concrete is classically considered to be.
Mathematical fact: 28 is the only positive integer that has a unique Kayles nim-value.
Trivia: 28 is the number of Chinese constellations, "Xiu" or "mansions" (a literal translation), equivalent to the 12 western zodiac constellations.
Historical date: January 28th is the day in 1909 that United States troops leave Cuba with the exception of Guantanamo Bay Naval Base after being there since the Spanish-American War.