Via Language Log:

Two years ago, the favored lunar New Year’s greeting in China was “Happy 牛 Year!” where 牛 (“bovine”) is pronounced niú in Mandarin and is standing in for “New” in the Year of the Ox / Bull / Cow. […]

What we have for the current year is “Happy New Year 兔 you”, where tù 兔 (“rabbit, hare”) is standing in for English “to,” hence “Happy New Year to You!”