I'm double-checking this information with the respective staff members and will get back to you soon.

Don't have a specific temperature for you. However, it looks like it will be somewhat cold. It is winter + after sunset so it will be cold. For an outfit, it will likely be a gloves + coat weather, maybe even hat and scarf if you want. It's not freezing cold, they won't be shivering if they're wearing a coat and you don't need snow coat/pant/boots or anything. The number of people in the room and the fireplaces etc will help keep it from reaching that stage. But yes, your character would likely want to dress warm.

@Lucia Fox Thank you! I wasn't sure whom I should send a PM to and I thought that the Errands FAQ is only for general questions (not for one specific errand); thank you for letting me know that next time it's OK to post there!

@Hjørdis Jensen Thank you for the detailed explanation! This certainly helps a lot. Also, thank you for your time reaching out to the team who plan this errand although you weren't part of this errand.