For many years,
the Roman Catholic church used the
Julian calendar, which has a leap year
every year that is divisible by four,
making the average calendar year
longer than the sidereal year and
causing the date of the first day of
spring to change gradually.
To correct this, Pope Gregory decreed that
Thursday, October 4, 1582, would be
followed by Friday, October 15. He
also declared that years divisible
by 100 would be leap years only if
divisible by 400.
For any year since
1582, if one printed two 12-month
calendars, one Julian and the other
Gregorian, with dates for the days
of the month, at least some of the
dates would not fall on the same day
of the week.
What is the first year
for which each day of each month will
fall on the same day of the week for
both calendars?
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