Mathematical fact: 7066 is the maximum value of n so that there exist 6 denominations of stamps so that every postage from 1 to n can be paid for with at most 13 stamps.
Trivia: 7066 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Historical date: May 6th is the day in 1840 that the Penny Black postage stamp becomes valid for use in the United Kingdom of Great Britain and Ireland.