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.
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...