In Western superstition, Friday the 13th is considered unlucky, but I thought it will be interesting to see if specific cultures have their own. I was inspired by the one that began today:
In Greek Orthodox culture, the period between 1st and 15th of August is considered unlucky. As kids, we were talk not to do anything risky those two weeks, because accidents are more common. Possibly has to do with Virgin Mary dying on the 15th. Random googling though says that August is also seen as unlucky in Brazilian, Italian and Hebrew culture.
Friday the 13th is actually known as Tuesday the 13th in Greek culture, because it rhymes better (Tuesday is "the 3rd [day]" in Greek).
Specifically in Cyprus, July is considered an unlucky month because a number of important negative events happened in July, and every new one just confirms the superstition.
On July:
-1426: Cyprus falls to the Egyptians
-1570: Cyprus falls to the Ottomans
-1821: The Archbishop of Cyprus is killed by the Ottomans to intimidate Greek nationalists (the Greek war of independence began on 1821)
-1878: Cyprus is transferred to the British Empire
-1974: The 1st President of Cyprus is deposed by a military coup - Turkey invades the island
-2002: A major peacetime military accident kills the upper echelon of the Cypriot Army.
-2011: Another major peacetime military accident kills 13, destroys the country's main power plant - coincidentally, the military base that was blown up was named after the Marshall who died in 2002 - same day too, you can imagine how that was seen as a confirmation of the superstition.
So in general, in Cyprus, we are conditioned to be wary of the time period between the 1st of June, and the 15th of August.