The carol is also unusual in that it contains both Christian and Pagan imagery, with holly and ivy both representing Pagan fertility symbols.
But according to old versions of the song, while holly was brought inside to deck the halls, ivy was left outside:
"Holly stands in the hall, fair to behold:That version of the song shows a contest between male and female symbols, with holly representing the male and ivy the female. It was also rather a sexist song, to modern ears, because it had chorus that went:
Ivy stands without the door, she is full sore a cold."
"Nay, ivy, nay, it shall not be I wis;However, as I pointed out last year, that is a little ironic because only female holly bushes have berries, the male bushes are quite plain. So maybe the women are secretly getting the last laugh in that contest after all.
Let holly have the mastery, as the manner is."
According to Cunningham's Encyclopedia of Magical Herbs,ivy is associated with Saturn and Bacchus. Both of these deities were honoured by the ancient Romans with festivals at around midwinter.
Worshippers of Bacchus and Dionysus carried a thyrsus - a staff wound with ivy leaves - in dances to honour their god. The staff is thought to represent the male while the ivy symbolises the female.
In magic, ivy represents fidelity in love. It can be used in love charms or carried by brides as part of their bouquet for good luck in marriage. If ivy grows around your house it will protect it and all who dwell within from adversity.
Cunningham's Encyclopedia of Magical Herbs