Chinese have been following the lunar calendar forever. It's based on the cycle of the moon around the earth. Our calendar, the Gregorian calendar, is based on earth's orbit around the sun. There are slight variations, so Chinese new year falls on a different day. This is more tradition now, since modern China mostly uses the western calendar. It is a huge holiday in China.
Gung hay fat choy is a new years greeting in Cantonese, which is the dialect most commonly spoken by Chinese immigrants who came from southern China, Hong Kong, and Taiwan. It is the unofficial language of Chinatown in San Francisco. Nowadays, school children in China are taught Mandarin (official language) and English. They would pronounce the words differently. The word characters (恭喜 發財) have the same meaning, but are pronounced differently in each dialect. There are well over 100 different spoken dialects in China.
Gung Hei = 恭喜 = congratulations
Fat Choy = 發財 = prosperous, become wealthy
Together it means "wish you the best of wealth"