The one day of the week where you can be dirty! lots of people do "dirty" things so why not a day named after it? Tuesday is said to be the dirtiest day of the week naturaly. The day where you can have sex of any type woth any person.
boy: Hey, what are you doing today?
other boy: Its Dirtytuesday so im doing my girl!
boy: Man, i love Dirtytuesday!!