I'm planning to run a Dickens' Christmas Carol equivalent adventure and wondered if there were resources about Deadwinter Day traditions in Faerûn to build on.
Since FR is polytheistic, there are very many different rituals and activities held on the Deadwinter Day, also known as Midwinter. A brief list of such activities can be found on the FR wikia site.
Regarding whether a Christmas-like holiday exists in the Realms, the answer is no, as you can read at the discussions on Candlekeep forums. Quoting from the "So saith Ed" section, ie. the collected answers of Ed Greenwood:
December 29, 2004: BrokenRulz, Ed hereafter makes answer to your question: "How does Elminster celebrate the Yuletide/Soltice?"
... such times there's some restlessness in Mystra, due to her origins (Selune), and the Weave is therefore more active than at most other times (...). Beyond such happenings, this time of year has little greater significance to Mystra and her Chosen. However, Elminster has visited our Earth often, and is familiar with Yule celebrations and the more modern commercial Christmas.
El has little use for presents and much of the foolishness associated with the etiquette of modern organized faiths of any sort, but it pleases his nature to do at Yuletide what he’s always done as a Chosen: help folk and support their belief in magic or strength of Sense of Wonder.