Friday the 13th is considered an unlucky day in Western superstition.
It occurs when the 13th day of the month in the Gregorian calendar falls on a Friday,
which happens at least once every year but can occur up to three times in the same year.
In 2017, it occurred twice, on January 13 and October 13. In 2018,
it will also occur twice on April 13 and July 13.
There will be two Friday the 13ths every year until 2020,
where 2021 and 2022 will have just one occurrence,
in August and May respectively.