Before Christmas no doubt xD.

September.

You get a launch boost and then a Holiday boost.

I heard summer isn't ideal for publishers since people do a lot of traveling then, so the assumption is they won't move as much units. As a result they aim for periods before and after.

When did Far Cry 3 launch again? I remember it didn't launch in time to be placed on the list of candidates for GOTY. It still sold well though, but studies suggest the GOTY titles helps move more units (which makes sense). It would be interesting to have this done under a controlled setting (if possible).

Well I would say near the holiday season for the nice boost, but you have to compete against the huge franchises during that time too.

Depends on the game. Kiddy games can launch during holidays no problem with competition. Huge games can as well as evidenced by skyrim, CoD, U3 combo. Medium games I think would be better launching in September like resistance. New ips can have a safer launch May through July. Seems to me as well that sequels often get released more January through April.

October is the best month to launch a game that is already from a developed series. (Ex: Mario/Zelda)
After your Launch Week/Month sales you'll have Black Friday soon after, then a slower period, then another explosion.

For newer IP's Late Spring/Mid Summer is most likely the best as there isn't a ton of competition.

before xmas seems little bit obvious, but i thin what is coming out at the time has ALOT to do with that too. like dont release a game when GTA5 comes out.

When the game is fully finished and there is less competition.

September/October time

Spring or summer. The period up to Christmas is usually overcrowded with the big games, so a new IP will struggle then. I wish it didn't matter though, i want new games all year round. There's nothing new coming out in July at all it seems.