You can obtain walk-in permits for the next day. I have done so. The best part is, they are free. The reservation fee for advance permits is 15.00/head. But of course there is no guarantee that any will be available. But historically a small party on a weekday has an excellent chance.

So......are you feeling lucky?????
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If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.