Well. the obvious connection is that Sunday is the main religious day here in England. and Friday is in Afghanistan(Islam). Query: can anyone Jewish tell us if Jews shouldn't cut their nails on Saturday? I feel that we may have something pre-Christian, and pre-Islam here, and very widespread: England and Afghanistan are a long way apart.
   Difficult to see any rationale for any problems with nail cuttings. Witches used your cut bits of nails to cast spells on you, it must be something on those lines, but why a partticular day?. You would tend to think it might be safer on Sundays or Fridays, with God around to help you, rather than more dangerous.Perhaps is too busy on Sun/Fri to keep such a close eye on small bits of fingernails. ll very odd. Any contributions from people of other religions/countries welcome.