The biggest traditional Chinese festival is Chinese New Year. This is typically celebrated on the first day of the first month in the lunar calendar, so it can range from January to February. This festival is huge: fireworks, paper dragons, traditional dress, moonpies (unless you’re actually in China, but that’s another topic altogether), etc.
For those curious, Chinese New Year is February 5th in 2019. In China, this festival is known as Spring Day.
Traditions vary widely depending on where you are when this is being celebrated. The day before might be time for a huge reunion feast with the family. Others might give money in red paper envelopes or sweep away bad luck with a ritual. It’s such a diverse holiday and celebrated in many places that it’s hard to cover completely here.