In reply to cheeky:
December is Winter. There'll probably be snow on the top. Some parts of the main Pony Track path are likely to be icy, especially higher-up. Could be very cold and/or windy.
I'd say walking crampons are definitely necessary at that time of year. Axes might not be necessary if you're sticking to the path, but at a minimum trekking poles (for stability) should be carried. Personally I'd definitely take walking crampons, axe, and poles and recommend you do the same.
Up and down the Ben is a long day out. In December you've got limited daylight, so everyone needs a headtorch, spare batteries, and should be comfortable walking in the dark. Obviously also appropriate clothing, food, flask, etc. Fitness to be on the hill all day.
You need to be able to navigate a dogleg route from the top of the track to the summit cairn. Easy enough in good conditions, but harder in the dark or in limited visibility. See https://www.ukhillwalking.com/articles/skills/navigating_on_and_off_ben_nev....
Some people tend to underestimate Ben Nevis because it's so near to a large-ish town and has a car park at the bottom. It's a serious mountain at any time of year. Have a fun challenge but be realistic and know when to turn back. Only you know your abilities.
Post edited at 15:52