Thursday

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.

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.

Carlisle

At Carlisle, towels and bathrobes are only worn on the last Thursday, called "End of the World Day."

Saratoga Springs

See Carlisle.