In multiple fanarts and fanfic websites of Full Metal Alchemist, I have seen mentions of the "503 day". Here's an example:
I did read and watch Full Metal Alchemist and Full Metal Alchemist Brotherhood and I do not remember any mention of such a date. It does not match with the date Ed engraved into his State Alchemist Pocket Watch either.
What does the "503 day" refer to?