Hecho general: 28 is the number of letters in the Danish and Swedish alphabets (not counting W), and also in the Arabic and Esperanto alphabets.
Hecho matemático: 28 is a harmonic divisor number, a happy number, a triangular number, a hexagonal number, and a centered nonagonal number.
Trivia: 28 is the number of grams in an ounce (approximately), and used as such in the illegal drug trade.
Fecha histórica: January 28th is the day in 1724 that the Russian Academy of Sciences is founded in St. Petersburg by Peter the Great, and implemented by Senate decree.