It glows at the end of three months (5)
I believe the answer is:
ember
I believe this clue is a double definition.
'it glows' is the first definition.
(I've seen this before)
'the end of three months' is the second definition.
(September, November and December)
'at' acts as a link.
(Other definitions for ember that I've seen before include "what causes a warm glow" , "Burning coal" , "Piece of live coal" , "showing residual fire" , "Remnant of bonfire" .)