Before I start, please not that a new year's bash is a party that can only be thrown if you have The Sims 2: Happy Holiday Stuff Pack or The Sims 2: Holiday Party Pack installed.
Make sure you throw the party between 7 pm and 8 pm, because otherwise the police will shut your party down at 11 pm.
You can invite up to 8 friends or neighbours to come over for the party. I decided to invite Mortimer, Cassandra and Alexander Goth, Nina and Dina Caliente and Don Lothario.
The Sims will autonomously start making ruckus by drumming on pots and pans. Other Sims in the room will join in. Around 11 pm Father Time will arrive at the lot. Your sims will have to call him over because he won't come in autonomously.
When he comes in he will shake his hourglass for a few in game minutes, before making ruckus. All the sims in the room will join him by drumming pots and pans, like they did when the party started. At 12 am he will change into Toddler New Year and after an Auld Lang Syne snippet he'll start dancing and slowly fade away. Then your party guests will leave your house.
A new year's bash can in fact be thrown every day, it doesn't have to be winter. So don't hesitate and throw that party tonight!
Unless you're too busy partying at you're own real life new year's bash!
Sulsul and happy simming!