Fatto matematico: 4187 is the smallest Rabin-Miller pseudoprime with an odd reciprocal period.
Trivia: 4187 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: June 18th is the day in 1264 that the Parliament of Ireland meets at Castledermot in County Kildare, the first definitively known meeting of this Irish legislature.