Holidays or not, I know plenty of people who save and look forward to the Steam sales all year long. They have huge deals on tons of video games,and for gamers, that's a priority over the holidays. My roomate got a gift card from his mom, and used it to by all Steam games.
The entire purpose of Steam is for easy access to games, both popular and obscure. If gamers wanted to buy a game that's on sale, it takes as little as five minutes to log on and purchase it. I suppose some people have very busy holidays, but none should be so busy that they can't find a few minutes to log onto Steam.
The holidays are a beneficial time for Steam to release a sale on its products. Many gamers are of school age, whether that be middle school, high school or college. The winter holidays are a time where these students are given a long reprieve from their schooling, and thus have more time to play games. Also, it is a time where their parents or loved ones are more willing them to purchase them a game (especially one on sale) as a gift.
Although the holidays may postpone shopping for some gamers, those who love gaming will be able to find time to take advantage of the Steam Holiday Sale. Steam runs the sale long enough, from December 18th until January 2, to give gamers plenty of time to find the games they want at a great price.