Bifil that in that seson on a day
‘Bifil’ means ‘it happened’ (We still, just about, have ‘befall’ in the modern lexicon.). So the Canterbury Tales happened in the beautiful spring time, in which he just described.
You shouldn’t need it if you’re reading Chaucer via Poetry Genius, but here’s a Middle English dictionary for reference.
To help improve the meaning of these lyrics, visit "The Canterbury Tales (General Prologue)" by Geoffrey Chaucer and leave a comment on the lyrics box