ranked

There will never be a real reset. I believe what they plan on doing is a soft reset, where everyone will be moved slightly closer to what once was 1200 elo (about silver V). If there was a full reset, people who were once in bronze would now be playing against some pros for a few games which would most likely not be enjoyable for them. However I do believe that league demotions will be implemented so all those boosted accounts wont stay in play 5, or gold 5 for long. As to when, I think that it will be some time in Febuary, but you know RIOT, always waiting to the last minute when everyone is freaking out to do things :D!

As stated previously, it will be a soft reset. Riot stated that your MMR would be set to something around the MMR of a "low silver player", though I assume it's more along the lines of your MMR being set to the average of your tier, or something like that.

As for when, it will be reset at the start of the 2014 season (Season 4), which should be around the end of this month, or mid-late February.