Being at an FI, I can see how a banking holiday could occur, but I don't see what the purpose would be. Atash is right, almost all transactions are electronic, and thus largely dependant on computer systems. Large core processing systems are difficult to change overnight, so if there were a banking holiday, it will be long time- 2 weeks or so. This would give FIs enough time to become compliant with whatever regulations are passed making the holiday necessary.
I do think that having a sum of cash stored at home somewhere would be best however, because cash is money in hand, and you can't buy anything when the phone lines are down or electricity doesn't work. Be prepared.