friday the 13th
when is it??
how often is it??
there can be 1, 2, or 3 friday the 13th's in a given year.
in a non-leap year, the possibilites are:
- jan → oct
- feb → mar → nov
- apr → july
- may
- june
- aug
- sept → dec
in a leap year, the possibilites are:
- jan → apr → july
- feb → aug
- mar → nov
- may
- june
- sept → dec
- oct
notice that there are only two changes from the non-leap year options to the leap year options:
- january leaves october to join the april/july pair.
- february leaves the february/march/november trio to join august.