I do this most years, during New Years eve, but only when the weather will allow it.
The all important thing to remember, is that if the weather is poor, then turn around, and come home.
I suppose i am in the fortunate position, of living in very close proximity to the mountain, so within an hour or so, i can be parked in llanberis, but i always look at the weather conditions.
Two years ago, the weather was so poor at the summit, during New Year, that many people were turning around at Clogwyn, thinking it too dangerous to continue.
Well, i did continue, using the railway as a visibility guide, and got momentarily lost around the back of the Hafod Eryri building, and had it not been for the adjacent railway line, which i knew was off to my left, heaven knows where i would have ended up.
Just look at the weather forecast, adjust your expected walking time, and ensure you have a good head torch.
Soldiering on, whatever the weather conditions, ive discovered, is not a sensible thing to do.
Just take your time, and if your still committed to reaching the summit, use the easier llanberis Path to the top.
Its longer, and will probably take at least three hours to the top, but its an easier route when navigating in the dark.
The thing about traversing a mountain as high as Snowdon, in the dark, is that it takes quite a bit longer to reach the top.
Your familiar with the route, but somehow your afraid that your going to stumble or fall, do to the poor visibility around you.
I normally reach the summit in around two and a bit hours from the llanberis, but in darkness, its well over three.
If its misty, then its particularly challenging, but just take care, and realise it will take you a fair bit longer to get up and down, than if your doing it in the daytime