Thrace hosts every year, festivals and carnivals where all the family can let their hair down and have fun.

let 是动词，意思是"允许、让"。 their 是形容词性物主代词，指代"他们的"。 hair 是名词，意思是"头发"。 down 是副词，意思是"向下"。 let one's hair down 是一个习语，字面意思是"把头发放下来"，引申为"放松、不拘束"的意思，因为过去女性通常把头发盘起来，只有在家放松时才会把头发放下来。