See: http://www.englishclub.com/grammar/prepositions-at-in-on-time.htm
– NoneNov 19 '13 at 15:41
1
You would use _in_, but if you added the day, the preposition would change to _on_: ELL opened **in** January 2013; the first ELL question was asked **on** January 23, 2014.
– J.R.Nov 19 '13 at 18:42
See my lovely answer [here](http://ell.stackexchange.com/questions/11041/in-vs-on-followed-by-a-year/11042) as well.
– Tyler James YoungNov 19 '13 at 23:19