"Spring Festival is the biggest festival in China."
In Chinese, adjectives with 2 or more syllables should use a 的 before the noun. The 的 is not required for single-syllable adjectives.
小鸟 = small bird
大节日 = big festival
聪明的鸟 = clever bird
最大的节日 = biggest festival
Here "biggest" is an adjective and "festival" is the noun.
When 的 is used for possession, it can be left out (e.g. 我狗 vs. 我的狗).
这是她的聪明的鸟 = This is her clever bird
这是她聪明的鸟 = This is her clever bird
So in Duo's sentence you could say:
The 的 for possession can sometimes be left out, but the 的 for a 2 syllable adjective is usually preferred or required.
I'm not a native speaker, but I'm pretty sure that a 的 can be removed if it would've been repeated multiple times. For example, "this is my favorite movie" would technically be 这是我的最喜欢的电影, but something like that would usually be written as just 这是我最喜欢的电影. In this case, the 的 can sort of apply to both the 我 and the 最喜欢, if that makes sense.