http://wiki.mabinogiworld.com/index.php?title=Patch

Here is a list of almost all changes made to Mabinogi and the accompanying season. This should be where you check first when you want to know when a feature is likely to be released. If it is not listed here, THEN you are more than welcome to ask on the forums.

Also,

Q&A bonus round!

Q) "I know it says it's released at (G?S?) but Nexon is releasing stuff out of order from time to time; anyone know when it will REALLY be released?"
A) No, we're all just as in the dark as you are, none of us are psychic enough to divine when Nexon is going to have an odd day and decide to switch stuff up.

Q) "I know it's released (G?S?) but when is (G?S?) coming?"
A) The best we can do is guess, no one really knows when any patch is coming as computer software is susceptible to delays for the oddest things, many times without notice.
The best estimate available is that Nexon tends to release one season per month; find how many seasons are between the current season and what you want, that's the closest you're likely going to get.

Q) "wen iz duel welding comin out?"
A) ¬_¬