Just out of curiosity, what does one do if his bar mitzvah falls on a fast day such as Yom Kippur or Tisha BeAv or any other of the 6 fast days? Is the seudah passed on to the next day? The previous day? Or is it done anyway on the day on lesser fasts?
Just like bringing a sacrifice, arranging a סעודת מצווה - a Mitzvah meal (feast) does not have to be on the same day.
It's not a Mitzvah on its own and there are no obligations to fulfill at that feast, it is a custom based on the Gemmorah in Kiddushin (31a):
"אמר רב יוסף מריש ה"א מאן דהוה אמר לי הלכה כר"י דאמר סומא פטור מן המצות עבידנא יומא טבא לרבנן"
The feast can be held on the same day, same week's Shabbos or any other date (or years later).