First of all, let me congratulate you on planning the activities a month in advance, which will ensure that whatever you do will be well thought out and executed.
Here are a few suggestions, and you can tweak them to suit your organization's style, employee strength, and budget.
1. Plan a Christmas carol evening. This might be unique for many who have not experienced it. One way is the simple way of getting in touch with a church youth group or a professional choir who would come and sing a variety of carols. Distributing the lyrics of the carols to the employees can get them to sing along.
A more interesting variation, which would also work as a team-building activity, is to get departments to sing some carols. With time on their side, it would be for them to find the right resource person to train them and, if there are musicians, to add the right beat and music too. You can make it into a competition too.
2. Have a Christmas tree on the premises, and employees can decorate it. Then follow it up with gifts for every employee, which can be given on Christmas Eve or New Year's Eve. You can give budgets to each department to source a variety of gifts, one for each of their staff. The twist is to ensure that no names are put on any gift and instead have someone (preferably an outsider who does not know any employee) choose a gift and pick a name from a bag that contains all the employees' names. Thus, no one knows what he/she is getting.
You can combine the above with the Carol Singing activity.
I am sure you will have a wonderful Christmas and New Year celebration with some more novel ideas being sent in by Citehr members.
Best Wishes.