what in the actual hell.

{quoted}

Actions that have been performed on your account are in violation of the League of Legends Terms of Use. You can review these terms at http://www.leagueoflegends.com/legal/termsofuse

First of all, because of the above bit, your best bet for a solid answer would be to send in a Support ticket. You can reach Support either through the button at the top of the screen, or this link.

Due to the severity of this violation, your League of Legends account has been banned until Tue Feb 22 22:22:22 UTC 2400

The date in this section is a code used internally by Riot to indicate why certain permabans are issued. There's no "official" information about this, but a long while back some people did a bit of research into what Rioters had stated, and came up with this list.

According to that, a 2400 code might be for impersonating Riot (using some variant of Riot in your name). That may not be accurate, though, or may have changed since the list was built, so I still recommend contacting Support to figure out exactly what's going on.

Only support will be able to give you the specific answer. As Jubbinaut said, in the past people have attributed this to impersonating a Riot-er. Over the past 2 years, 2400 has been used during Refer-a-friend frauds, account sharing, and impersonation bans (checked on reddit and the boards googling "Riot bans code 2400").

So, if you've ever used refer-a-friend to obtain RP in a devious way (IE, a smurf), shared your account, or impersonated a Riot employee, they could be acting on those.