The tradition in corporate companies that don't require full formal attire every day of wearing a tie exclusively on Wednesdays.
It can signify a number of things, from a passive aggressive display of superiority, to remembrance of a former company (where ties were required daily), to "I have an interview and want to cover it up by claiming participation in a quasi-tradition".
This is not to be confused with Thai Wednesday -- the tradition of going to a Thai restaurant on Wednesdays. Though the two can be used together in a kind of double entendre.
Colleague: Will! It's Tie Wednesday. How could you forget to wear a tie?