From Halopedia, the Halo wiki
The Humpday Challenge was a weekly game which takes place between Bungie Studios employees and large Halo fan sites, active communities, and professional teams. Humpday Challenges always took place on Wednesday.
Numerous communities hoped to challenge Bungie Studios, but few are selected. To be selected by Bungie for a Humpday Challenge is a challenge in itself. Bungie Studios does not take skill into account when choosing opponents, rather they pick teams to play based on community activity. Other companies or groups that has some kind of niche are picked most often. The Company does not seek to play the best players, but in gaining mutual publicity from the coverage that a Humpday Challenge Brings. As such, it's usually just players the administrators or community leaders of its picked websites.
Bungie has regularly played various groups during the challenge, and some have had the honor to play the company twice. A list of opponents Bungie has played includes: