I've laid out our itinerary so that we'll be visiting Stonehenge on New Year's Eve (during the day, or course). If you have been to Stonehenge on New Year's Eve in the past: How crowded was it? (I'm wondering how far in advance to buy my online tickets.) Thank you!
I can't imagine it being busy = all the staff will be wanting to go home,
can't see the attraction of standing in a field in the middle of winter
If you know that you want to visit, why not buy tickets now?
How busy it will be will depend on the weather. Schools are on holiday, but many people will have other things on their minds on New Year's Eve.
Are you driving there? Public transport maybe limited.
It will be cold and windy. Might be raining too. Go early. The staff will want to go home at closing time.
It gets dark around 4:30, so plan accordingly.
The henge itself is very much exposed to the weather, but there's also the excellent visitor centre, which has exhibits indoors as well as out. And the fairly large gift shop and cafeteria.