7 on the 7th
From Halopedia, the Halo wiki
7 on the 7th is a playlist only available on the 7th of each month. The 7 on the 7th playlist exists to ensure that people have an opportunity to get the Seven on the Seventh Vidmaster achievement. The requirements for this playlist is, Cold Storage, the Legendary Map Pack, the Mythic Map Pack and the Mythic Map Pack 2.
When playing, your service records will having a record saying "7 on 7th EXP" meaning that each time you win in this playlist (1st, 2nd, or 3rd place) you will get one point recorded here. If you get 7 EXP points in total for this playlist, you will achieve the vidmaster.
Assembly 7 on the 7th
Avalanche 7 on the 7th