How do these attributes work together

Question

For full context: http://www3.nhk.or.jp/news/easy/k10011281951000/k10011281951000.html

The sentence in question: ８日は、２０歳になって新しく成人した人をお祝いする成人の日でした。

My attempt at translation: "About the 8th, it was the day of adults..." I can't really translate any further. My translation makes so little sense then it gives me a headache.

The particle を requires a direct object, and this object would have to be お祝いする。If I regarded お祝いする as an attribute of its own to 成人, it couldn't be interpreted as an object in context of the ２０歳になって新しく成人した人を clause anymore. However, if I don't regard it as an attribute of 成人, then I could theoretically interprete it as the noun お祝い representing the direct object in the ２０歳になって新しく成人した人を phrase. Then する would be the predicative of the ２０歳になって新しく成人した人をお祝い phrase, which seems possible in theory, but then again feels wrong because I've never before analysed a する verb that way. Rather I regarded them as one syntactical unit amounting to a predicative in the respective clause. Anyway, even in this latter case I just described, the translation I could come up with doesnt make much sense anyway and also probably doesn't even correctly reflect the syntax I assumed: "About the 8th, it was the day of adults who gift newly grown up adults becoming 20."

Answer 1

８日は、２０歳になって新しく成人した人をお祝いする成人の日でした。

In it simplest form this sentence is

８日は成人の日でした。
The 8th (of January) was 'Coming of Age' day.

The rest of the sentence is a relative clause modifying 成人の日. What kind of day is it?

人をお祝いする成人の日
A coming of age day where we celebrate people.

What kind of people do we celebrate? This is another relative clause modifying 人:

２０歳になって新しく成人した人
People who became 20 years old and newly achieved adulthood.

Overall:

８日は、２０歳になって新しく成人した人をお祝いする成人の日でした。
The 8th was Coming of Age Day where we celebrate people who have become 20 years old and have newly reached adulthood.