Thursday
Thursday is the fifth day of the week. According to some systems, it is the fourth day of the week. Twice a year, at Lancaster, it is actually the sixth day of the week, which, according to the said odd systems, makes it the fifth day of the week. Using poor logic, on average, Thursday is the fifth day of the week. Which would be true anyway.
According to The Hitchhiker's Guide to the Galaxy, the Earth is destroyed on a Thursday, and the only survivor from Earth on the day of destruction is a one Arthur Dent, who happens to be wearing a dressing gown. He is saved by his friend, Ford Prefect, an alien who knows how to hitchhike throughout the galaxy, etc.
Lancaster
Hence, each Thursday, CTYers wear bathrobes all day, for fear that the world might end. Those who don't have bathrobes carry towels, the most useful item an intergalactic hitchhiker can have. Those who do have bathrobes bring towels anyway.
Also, many CTYers refer to the last Friday of camp as Thursday since the CTY (which is the world) both end. The last two days are switched at Lancaster (i.e., Monday, Tuesday, Wednesday, Friday, Thursday). This is also sensible in the fact that the dance now occurs on a Friday; we can't have dances on Thursdays. Therefore, the last calendar Thursday is "CTY Friday" and the last calendar Friday is "CTY Thursday".
Some have taken to calling Thursdays "Bathrobe Day;" however, most people understand that you just can't not have a towel or bathrobe on said day.
Bathrobes
At Lancaster, those who do not wear their bathrobes on are said to be "naked."
Groups of people had worn bathrobes since 1992, but the connection with this and the modern-day tradition is unknown.
In 1995, Dennis Clark became the first person to wear a bathrobe on Thursday. Dennis had a wonderful Dennis-colored (blue-green) bathrobe that he would wear on Thursdays.
A particularly striking bathrobe story concerns B-B Stern, who attended Lancaster in the late 1990's and early 2000's. B-B apparently wore his bathrobe to school during the rest of the year as well, and the tradition was adopted by a friend of his, Sean Treanor, who had not attended CTY. Sean then continued to wear his bathrobe even after B-B had transferred to a different school. It is unknown whether the bathrobe tradition has spread anywhere else outside of CTY.
--Sammka 21:19, 27 August 2006 (MST)
The biggest Hoopy Froods wear British bathrobes.
Carlisle
At Carlisle, towels and bathrobes are only worn on the last Thursday, called "End of the World Day."
"As far as I know, this has been around at Carlisle since well before '95. It was originally called "towel day" due to Ford Prefect always carrying a towel with him (and the million other significances towels have to the hitchhiker's guide). Towels were worn in numerous creative manners. At some point, some people combined it with bathrobes (these people couldn't cope with towels), and so it is referred to as "towel/bathrobe day" or sometimes (sadly) just 'bathrobe day.'"
--SomnusLupus 28 Nov 2004
Edit: I believe this tradition has largely ended as of '06, as Carlisle tries to differ itself in tradition from Lancaster and many believed that this was a cross-over tradition. It has also come into conflict with Goth Day in the past (second tuesday) which caused Goth Day to largely not happen.
In Carlisle '06 2 there was an impromptu "toga/bedsheet day" during the second week. I'm not sure if this was a tradition or just a bunch of people who decided to wear togas and tell everyone else it was "toga day".
Also, every Thursday you are offered free high-fives becuse every Thursday is free high-five Thursday!
Saratoga Springs
Last Thursday is known as End of the World Day, on which it is traditional to wear a towel.