According to a post on Yahoo Answers, 'Stay Another Day' is a 'Christmas Classic' because 'they dressed like Yetis'.
It's kind of a shame that they didn't adopt the yeti fashion as a permanent look. It would certainly have made them distinct from their early 90s boyband peers Not that East 17 really looked like a boyband. They looked like someone had selected four random men from the nearest scary boozer and offered them popsuperstardom.
Of course, it didn't happen quite like that. Tony Mortimer, the band's songwriter (and also the one with the talent for looking slightly less silly than everybody else) originally had a solo record deal. And then a teeny- bop supervillain decided he needed a vocalist (hence Brian Harvey with his decent choir boy voice and unpop teeth) and some 'dancers' (hence 'the other two' trying to geezer things up with their shaved heads and hip-hop hand gestures.)
The dramatic tale
of East 17 makes a good sad story for Christmas. The article I've linked to hints at George Bailey style redemption, but in fact the reunion tour was not a great success. East 17 are still going without Tony. They have recently released a song full of swears and apparently did a late night show in one of Glastonbury's many dance tents earlier this year (which I missed because I was fighting a losing battle with a leaky sleeping bag).
Still, everyone likes 'Stay Another Day' and it has been covered by both Girls Aloud and Glasvegas. The song is basically a young man's attempt to deal with sadness. It also works as a break-up song, with (Christmassy) bells on.
The song also had a non-festive video, which features the band looking thoughtful around a piano, but frankly the white parkas are more memorable.