It was after college, when I was in my first year of architecture school, and miserable, that Damon and Dean started talking about reforming a band. I thought that sounded like fun and volunteered to play bass
Naomi Yang - Temperature's Rising: Galaxie 500 an oral and visual history (p16)

Galaxie 500 was born the day that Naomi volunteered to play the bass guitar
[…]
You can spend your time placing ads in the Village Voice and sifting through messages on your answering machine by idiot musicians […] but the best thing is to start a band with your friends. Your friends are tasteful and smart and like the same things you do. Who cares if one of you doesn't play an instrument? She can learn.
[…]
We had our first rehearsal on May 27, 1987
Dean Wareham - Black Postcards: A Rock & Roll Romance (p33)

MP3: Pride - Galaxie 500 (1987 - extended demo cassette)

* OK, there's a receipt for Naomi's bass guitar in Temperature's Rising that's dated 2nd May 1987 which suggests I may have to revise Galaxie 500's birthday... AGAIN!