From here it was taught: One may fulfill his obligation [of rejoicing] with meat of voluntary sacrifices. I might have thought [that one can likewise fulfill his obligation] with fowl and meal-offerings? Therefore it states "Only..." [to exclude this].
(Devarim 16:15) "Three times in the year": "times" signifies "festivals," as it is written (Isaiah 26:6) "the times" (i.e., festivals) of the poor," and (Song of Songs 7:3) "How beautiful are your (festival) times (of "steps" [to Jerusalem]) in sandals!"