it's not a specific date when it comes but bungie just do random games for each weekend and sometimes it could be grifball so every weekend just check it out if it's the game you want to play, but if it's not there just download the link from bungie, having the map and gametype and play grifball with your friends in custom games hope this helps :P
so far i have only seen it in a doubble xp weekend playlist.
Grifball is a Double EXP Weekend playlist on Halo 3 matchmaking. It resembles football, but with bombs, swords, and Gravity Hammers! Grifball can be played in Halo 3's online multiplayer whenever it is featured as a Double EXP Weekend playlist.
The double exp. weekend games are fiesta, grifball, infection, three ball and rocket race. I feel like I'm missing a few, so update this if I am wrong. Thanks!
Halo 3 grifball is a popular gametype in Halo 3 online multiplayer which resembles football, but with gravity hammers and swords! Grifball is only available as a Double EXP Weekend playlist and is featured about once every month. Grifball has become so popular that people have formed teams and play in tournaments. So get out there and play Grifball!
After the end of the of the double XP weekend
This Weekend. September 7th - 10th.
Treyarch will make an announcement a couple days before double XP weekend, but when they do gaming websites are good to look it up (gamespot, IGN, G4tv,…)
On Xbox they do this a lot of times. But on PS3 it hardly happens.
you will get more points then usual
there randomly selected week ends, usaully they do them for holidays so maybe the weekend of christmas.
its coming in 6-7 th march
its a double XP weekend where the halo fans of the world fight the bungie employees in random gametypes