Ah yes, a common entry title these days. I'm not sure what exactlys supposed going on during the solstice, but from what I can gather at least two things are supposed to happen:
1. SlenderMan will be weak
So... He'll be weak. Then what? By my reasoning, even if hes weak He can still slenderwalk, and, He still has an army of Slenderproxies. So, let's say you did get close enough to try and hit Him, would it even do anything? Past attempts to harm him have almost all ended in failure, so whose to say this time would go any better? Then again, I may be completely wrong and someone might end up killing Him.
Another question is, why? Why is He going to be weaker on the solstice? Is He a firebender or something? Does this mean that all the waterbenders will be stronger? It doesn't make any sense.
Then again, nothing about SlenderMan makes sense.
2. A hero will arrive
And what, is the hero gonna snap his fingers and destroy Him? I highly doubt it.
I'd be more surprised if the hero actually did anything, considering the points I made in 1. He might try to fight Him, but all our hero would have to do is blink and He could slenderwalk halfway across the world. The hero might not even get the chance to fight Him, he might be met with an army of Slenderproxies bent on destroying him. Or perhaps, all the hero will do is show up.
Maybe nothing will happen, maybe everything will happen, no one knows, and no one will know until the solstice comes.