Friday the 13th is considered a day of bad luck in English-, French- and Portuguese-speaking countries around the world, as well as in Austria, Germany, Estonia, Finland, The Netherlands, Belgium, Republic of Ireland, Poland, Bulgaria, Denmark, Iceland, Sweden, Norway, Czech Republic, Slovakia, Slovenia, and even the Philippines . A British Medical Journal study has shown that there is a significant increase in traffic-related accidents on Fridays the 13th. Will you stay at home tomorrow? Why Friday the 13th Is Unlucky. .