That all depend on the pairs we are trading with, like you said that thursday and friday works for EUR/USD, and like wise we have different trading days for different pairs.
There are many days in which the market is volatile and provides opportunity, so it all depends upon you as a trader.
I see this fine thread’s still steaming ahead, replenished with ever-increasing insight and profundity. Actually, I have yet another thought to add to it: I’d stay away from bank/public holidays, myself … 
Yes, the insights of some are truly deep
It is hard to determine which day is best for trading as we all know the market is very intense and unpredictable so it all depends on the instruments. However Friday could be the day that most traders avoid to do some real actions (I’m excluding the NFP here) and of course the bank and public holidays. Monday is a bit slow as well s I think that Tuesday, Wednesday and Thursday are the most preferable days unless you have a long-term strategy.
There’s your best advise… Time of day is far more important then the day you trade.
I am fundamental trader and to be honest days like first friday of the month I am always away of trading. The NFP can really affect your account specially if you are trading in GOLD or EURUSD. Obviously I respect that others could tell me that also is the best moment to make your profits if you are trading in news but… I would recommend to avoid really big news. (is just my opinion)
For me the best days are: tuesday/wednesday/thursday.
Mondays are usually slow (at least till US session), as for fridays they can be very unpredictable due to profit realization/position closure before the weekend.
I would agree with you guys here, but I feel it still depends largely on the time in those days.
Most forex traders are more successful during the late US, Asian or early European trading sessions – essentially 2 PM to 6 AM Eastern Time (New York), which is 7 PM to 11 AM UK time.
Monday to Thursday ?
Rest from Friday to Sunday