Sound conclusion, mostly, about vulgar festival (9)
I believe the answer is:
halloween
'festival' is the definition.
(October 31st festival)
'sound conclusion mostly about vulgar' is the wordplay.
'sound' becomes 'hale' (both can mean healthy).
'conclusion mostly' becomes 'en' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'vulgar' becomes 'low'.
'hale'+'en'='haleen'
'haleen' going around 'low' is 'HALLOWEEN'.
Can you help me to learn more?
(Other definitions for halloween that I've seen before include "special night" , "annual celebration" , "October 31" , "autumnal celebration" , "autumn night" .)