You won't find a problem with ATM's in Porto. They seemed to be on every street corner. I
would agree that you should take out money while the bank is open but if you do loose your
card on the weekend cancel it a immediately. About a year ago I lost my visa card in an ATM
on Sunday and by Monday after I had spoken to the bank to see it was there they had run it
up for the tune of several thousand dollars. Somehow they got it out of the machine and while
I cleared it all up with the bank it is better to be safe than sorry.
Have a great time regardless.
Tom, London, Canada