Plot: Happy Halloween! It's no secret that Halloween is Ellen's favorite holiday, and this year her celebration's bigger than ever! Tune in to see the costume that everyone's going to be talking about tomorrow! She'll be joined by two of her favorite people on the planet! First, it's NEIL PATRICK HARRIS, star of "How I Met Your Mother" and the new special, "A Very Harold and Kumar 3D Christmas. " He always knows just how to crack Ellen up