From Brian May.com via Queen Vault.com, two quotes from Brian on the making of the two Christmas tracks:
Brian May: "I always liked the record ... it's something Roger and I did a rare early collaboration on. The original idea came from Roger, who had written that very shivery verse pattern. I had another idea too at the time .... it became Anita's Christmas single ... "I Dream of Christmas". But we judged the embryonic TGIC to be most suitable for a Queen track, and worked on it together - I added some tinsel, and the chorus idea, introduced by a modulating cascade of icicles, and it was completed pretty quickly. Nice and simple, and Freddie made a lovely job of singing it. I promise we will make a video for it one day ... the fact that there is no video for TGIC has of course always limited its exposure in these days of Video Supremacy ... We have actually been speaking about it ... but got too busy to manage it for THIS Christmas. So this is really good news ... it makes us happy that you have given the track new life."
"This is years ago, of course. Roger and I agreed we would both submit ideas, and work together on working up the idea we chose. Roger wrote the beginnings of "Thank God It's Christmas", and I wrote the beginnings of "I Dream of Christmas". We met down at Roger's place and demo'd both tracks together, as I remember. How the choice between the two tracks was made I don't clearly remember, but I think it was a whole group thing. We decided to finish TGIC for the Queen Christmas single, and "IDOC" was put to one side, and I immediately worked it into the version which we released as my Lady Anita's Christmas record. This record charted briefly but is now a rarity (it features Anita singing, along with a school choir of young kids, and quite a lot of Guitar soloing by me!) For the eventual Queen song, Roger had the verse form already: "Oh - my love - we live in troubled days ....". I contributed the chorus, with a key-change leading into it, using a lyric line which occurred casually in the verse, so this now became the title, "Thank God It's Christmas". Of course when Freddie took up the singing it jumped into another gear ... his lovely high register soaring above the background in these choruses, then returning to a very gentle breathy texture for the verses. The background was based on a synth pad which Roger had found (he would have to tell you what it was made with), to which I added a guitar texture very un-typical of me ... very clean and bright ... the amp turned way down and the strings hit hard like bells. This makes that Bright Frosty sound which I actually like a lot, contrasting with the warm and dark synth chords. Of course as usual John's subtle firm bass lines are a big part of the style of the eventual outcome. The drums are programmed, the way Roger originally demo'd them, an unusual choice for a Queen track but it works in this case. My favourite part is the end, where there's a small building (subdominant) link into the playout. Freddie is improvising, and it all becomes totally joyful ... trekking off into the distance across the snowy fields..... noses red, scarves wrapped around chins, snowballs being thrown .... Christmas packages of good food being delivered to hungry villagers' doors ... mulled wine being swigged to keep the blood flowing to those cold extremities.... "