Beerday was originally concieved by the CWRU CDMS group. From the original beerday page:
To celebrate the successful completion of CWRU Run 1, Prof. Daniel Akerib purchased beverages of choice for each person involved. Although most people chose Champagne, Mark Jackson of Oxford Instruments chose Sam Adams. Because you can't buy a single beer, a six pack was purchased, and the single extra beer after the festivities was left in the office of room 101. It set there for a while, until one day we realized that it didn't belong to any of us, but we had no way of getting it to Mark Jackson. Therefore, we drank it, and proudly placed it on our window. We then decided that this first "Beer Day" should become a tradition, and it has.
In time, most of the original Beer Day attendees have moved on from their days at CWRU. So the tradition doesn't die with time, it has been decided to start anew, in the "Basement" of Brown's Physics Department.