It is not closed till the 15th of the month it is closed THROUGH the 15th of every month. Which means it is open the 16th through the end of the month, whatever that is for that month.
Have made a couple trips out there on the 15th of a month only to find the gates closed.
You can go in through Black Rock Canyon which is going in the same way as Nightmare, just make a left at the top of the hill instead of a right. You will end up where Nightmare exits out, then down the hill. To the right takes you to last chance, to the left takes ya up to the tunnel and other sites to see.
Can't get lost on the black rock trail, off shoots have been closed, make no turns and you will get dumped out onto Last Chance. IF, my air pump was back from repair I'd be happy to give ya a tour of the place.