Halo 3

Since the TU2, each playlist has it's own ranking system which contributes to an overall ranking system - which older gamers may remember it being one global EXP system.

However to gain this you must have 7 EXP in any one of the playlists on the 7th of a month - all you need to do is make sure you load the playlist so that you are in the lobby - you don't even have to play the match.

Admittedly yes, for 0G this does seem a little pointless but apparently there is a rumour that there is a prize rewarded to those who have all of the Vidmaster achievements!

Have 7 Exp in a particular Playlist (Team SWAT, Lone Wolves, etc) then enter the Matchmaking Lobby when YOUR Xbox shows that it is the 7th of any month.

Exp, IS NOT the little number next to your Rank Insignia, that appears on your coloured GamerTag strip in the Lobby, that is your Skill Level.

Exp IS the small number (next to Exp) that you can only see if you "Green Button" the above coloured GamerTag strip while in a Matchmaking Lobby.

Method 1 (on the 7th of any month)
You do not have to even play a single game (if your Exp is already 7) to unlock the Achievement. Simply enter the Matchmaking Lobby screen, select the Playlist (eg Team SWAT) that you have 7 Exp in and pop "there she blows".

Method 2 (on the 7th of any month)
Get to about Exp 6 in a Playlist, then play that Playlist until your Exp raises to 7, the Achievement will unlock in between Games.

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I wish someone had spelt all that out for me,
so I posted the above for those who are/were just as confused as I was.

Best of luck out there guys...

Bungie has a "7 on the 7th" playlist available on the 7th of the month. All matches are 7 players, and the top 3 spots each earn 1 EXP. Therefore, you can get this achievement simply by finishing in the top 3 seven times.

You can boost this by getting a group of 7 people together and playing, rotating wins until everyone has 7.

You can also plug in extra controllers to fill the slots, but that may result in Bungie becoming Very Angry with you if those other controllers just sit there for the whole match. Don't recommend doing that.

Every month, there is a playlist that comes out on the 7th. This playlist is available for only one day. Get 7 exp points by winning 7 matches.

You should get this achievement fairly easily.

If you are having trouble getting this achievement and say your exp in one particular playlist is above 7. Like say your exp is 8 in lone wolves. You can quit out of a game early. You should lose 1 exp after. You may have to do it a few times like I did. But eventually you should get to 7 exp in that playlist. But a word of caution: you could ruin the game for some people. Hope this helps.