Hiking Mt. St. Helens is a very popular activity.  In fact, it is so popular climbers are limited to 100 a day between April and October.  Climbers must buy a permit if they want to climb during those months and practically the entire summer is already sold out.  Luke Whitfield with the Mt. St. Helens Institute says when the tickets go on sale February 1st, they go fast.  

"July is completely sold out, August is completely sold out, there are only a few permits left in June and September that are mid-week that are left." says Whitfield. He says there are plenty of permits left for October, if you don't mind the rain.

Of course, plans change, so there is a resale website for permits.  On purmit.com you can sell permits you can't use.