Interestingly, in contrast to many other countries, in Spain (and apparently in Latin America countries and in Greece) not Friday but Tuesday the 13th is considered to be an unlucky day.
I tried to find out the source of this superstition but Spanish Wikipedia states only that according to the legend it was on the Tuesday the 13th that languages got mixed during the construction of the Tower of Babel. According to which legend – remains a mystery. Another version is even more implausible — the day is unlucky because 13 was the number of apostles during the holy supper and Tuesday because it was the day Constantinople was sacked by the Ottomans. I am pretty sure even more tragic and disastrous things happened on Tuesdays, but that can be said of any other day as well.
Even more interesting is that in Italy the unlucky day is the Friday the 17th. Italians go as far as omitting the 17th row in their Alitalia planes.
In China and Japan, four is the one unlucky number. 9 is also not very lucky, but I guess they can live with 9th floor.
What I suggest is that we should be much more culture-sensitive in our globalised age. Superstitions are after all are an important part of any culture. Lets built hotels without 4th, 13th and 17th floors and ignore that room numbers as well, omit them in seat numbering in buses, trains and planes and provide paid day-offs for everyone on the 4th, 13th and 17th day of every month. That is the least what I demand.