Fatto generale: 1555 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1555 is the largest n so that Q(vn) has class number 4.
Trivia: 1555 is a boring number.
Data storica: April 3rd is the day in 1860 that the first successful United States Pony Express run from Saint Joseph, Missouri to Sacramento, California begins.