This article mainly tells us that the four types of festivals and how people celebrate.这个句子中的that是多余的,因为 the four types of festivals不是从句,所以不需要连接词；
而how people celebrate后面应该加代指 the four types of festivals的人称代词them作宾语；how people celebrate them是宾语从句,how是它的连接词.
the four types of festivals、how people celebrate them用and连接,作谓语动词tells的直接宾语,us是间接宾语.
新年快乐!