Friday the 13th
The fear of Friday the 13th has been called friggatriskaidekaphobia (Frigga being the name of the Norse goddess for whom "Friday" is named and triskaidekaphobia meaning fear of the number thirteen), or paraskevidekatriaphobia a concatenation of the Greek words Paraskeví (meaning "Friday"), and dekatreís (meaning "thirteen") attached to phobía (meaning "fear"). The latter word was derived in 1911 and first appeared in a mainstream source in 1953.
Tuesday the 13th
In Spanish-speaking countries, instead of Friday, Tuesday the 13th is considered a day of bad luck. The Greeks also consider Tuesday, and especially the 13th, to be an unlucky day. Tuesday is considered to be dominated by the influence of Ares, the god of war. A connection can be seen in the etymology of the name in some European languages The Fall of Constantinople to the Ottomans happened on Tuesday, May 29, 1453, fact that strengthens the superstition about Tuesday. In addition, in Greek the name of the day is Triti meaning literally the third day of the week, adding weight to the superstition, since bad luck is said to "come in threes". (Although some believe the first day of the week is Monday since the seven day is consider "the day of rest" and that day is Sunday - you do the math!)
Friday the 17th
In Italian popular culture, Friday the 17th (and not the 13th) is considered a day of bad luck. In fact, in Italy, 13 is generally considered a lucky number. However, due to Anglo-Saxon influence, young people consider Friday the 13th to be unlucky as well.
Have a great day and weekend!