This tip is written by Carol King: "Do take advantage of themed ad units; these units are specifically tailored to users in the appropriate locations as determined by their IP address. These ads have special colors and graphics to celebrate the season! For instance, a user in the United States might see colors and graphics appropriate to the Fourth of July while users everywhere could see colors and graphics for New Years Day". Get prepared.
Yes, but it's a pain in the bollocks to place new ads for only one day - especially if you've got dozens of sites to play with!
Ad units usually fit to general site theme, this could be difficult to rebuild all site only for one day (yes... css and etc... but as above said, with many sites...).