Halloween kicked off last night -- with lavish costume parties on both coasts!

Janet Jackson, Rachel Zoe, Tara Reid, Anne Hathaway, Ciara and Padma Lakshmi all celebrated in NYC, as "True Blood's" Ryan Kwanten and The Situation got their party on in Las Vegas.

We'll have a ton of new photos for you tomorrow -- as AnnaLynne McCord, Heidi Montag, Audrina Patridge, Kellan Lutz, Marilyn Manson and more are all hosting bashes tonight!