If the image below is to be believed, then the first Halo 4 Map Pack, Crimson, will launch on December 10th.
The ad is from the German dashboard and was spotted by a halocouncil member before the ad was pulled. The ad also gives the release dates of the other two map packs, Majestic (February 25th) and Castle (April 1st).
We already knew that 343 were releasing maps under these names so this information looks to be pretty solid. So gamers who purchased the Limited Edition will get these for free provided they punched in their codes, those who didn’t but want all 3 map packs can get a Season Pass.