Are tuesdays the cheapest day to book a vacation?

Yeah, in general, traveling on Tuesdays and Wednesdays is the way to find the cheapest deals for flights, hotels, rental cars, etc.

Update: oops--I don't think I read your question closely enough. If it's about booking, then I'm not really sure. My personal experience has shown Saturdays to be good, but that's not based on anything scientific. I really don't know if there is a best day to book. Good question...

GREAT ARTICLE
The days of the week you choose to depart and return make a difference in overall price as well. Historically, Fridays and Sundays are the busiest days of the week to travel, which also makes them the most expensive. In general, you can find lower fares by traveling midweek, on Mondays through Thursdays. If you must travel on the weekends, Saturdays are generally the most affordable option.

http://www.smartertravel.com/airfare/Airfare-Part-back-basics.html?id=13923

Top 10 Tips for getting the Cheapest Airfare
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Finding Low Airfares Online
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From time to time we find a travel-related site that has the potential to add more value to your travel. We don’t necessarily endorse them, but we think they are something to be explored.
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EP=Epcot,
DS=Disney Hollywood Studios
AK=Animal Kingdom
MNSSHP=Mickey’s Not So Scary Halloween Party (link)
MVMCP=Mickey’s Very Merry Christmas Party (link)
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In the travel industry, a week immediately following a holiday period—such as right after Thanksgiving or right after New Year’s—is known as a “dead week.” If your dates are flexible and you can time your vacation to coincide with a dead week, “you can save big money,
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The top three destinations in 2008 will be Las Vegas, Orlando and Los Angeles, according to ARC data.
http://www.travelmole.com/stories/1125104.php