I think it's there because the actual solemnity is kept on December 27th which is one of the great Christmas days. St John is so strongly associated with Christmas because his masterly Gospel has those wonderful words in the first chapter which sum up the whole of Christian theology
and the Word became flesh and dwelt among us
God becomes human in the form of Jesus Christ. However good old John, who is the patron of this parish and district deserves commemorating in his own right...so this strange feast...ante portam Latinam. Relates to a traditional story (story) where the emperor Domitian attempted to deep fry St John ...there is still a church called St Giovanni in Oleo (St John in Oil!) today.
I choose by and large to ignore it.
Last year, though, Sophie and I did have chips at lunch time!! But this year Kate and I refrained from eating any fried food.
Instead we had lovely steamed dumplings at the Market Food Hall