The number 13 on its own is not an unlucky number in Greek culture. The opposite is often  considered true by many Greeks, that is that the number thirteen is considered to be lucky.  Some areas in Greece say that the number 13 represents the 12 apostles and Christ with Christ being the 13th member.

The number 13 cannot be that unlucky as we should not forget that Greece won 13 medals in the Sydney Olympics its' best showing ever!

In most western cultures Friday the 13th is considered an unlucky day, to Greeks this day is not considered unlucky.  Greeks who have accepted this as an unlucky day are Greeks of the diaspora who have integrated a non-Greek superstition into their superstitions.  

In the Greek culture it is Tuesday the 13^th of the month which is unlucky.  It is the combination of the date "Tuesday"  with the number "Thirteen" that is considered very unlucky to the Greek people.