general_fact: 1573 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
math_fact: 1573 is a value of n for which n, 2n, 3n, and 4n all use the same number of digits in Roman numerals.
trivia: 1573 is an unremarkable number.
historical_date: April 21st is the day in 1922 that the first Aggie Muster is held as a remembrance for fellow Texas A&M graduates who had died in the previous year.