請問The day which Joseph shall never forget is the one when he joined the army中，the day後面為什麼接which，又為什麼the one後面接when，那which及when分別又是什麼詞性？

A more concise sentence should be:
Joseph shall never forget the day when he joined the army.

"the day" is the object of the main clause.

"when he joined the army" is a relative clause decribing "the day" (or "the one" in your sentence), where "when" is a relative adverb (= on which).

The relative pronoun "which" can be used in the above sentence as:
~ Joseph shall never forget the day on which he joined the army.

To emphasize "the day", you can expand the main clause by putting "the day" in the beginning of the sentence, connecting it with another relative clause and ending the clause with a pronoun "the one" as the subject complement.

~ The day [1st relative clause] is the one [2nd relative clause].

As "the day" is the object of the main clause, you can use either "which" or "that" or even omit the relative pronoun.

~ The day which Joseph shall never forget is the one ...
~ The day that Joseph shall never forget is the one ...
~ The day Joseph shall never forget is the one ...

All 3 sentences above are correct.

1.首先需觀念釐清，無論是關係代名詞、關係副詞、介繫詞、連繫動詞，均是將前後字句做連結
2.本句which是關係代名詞除將The day 及Joseph shall never forget連結並做forget受詞，例如：The man whom he talks to is my brother. 其次when是關係副詞除將the one 及 he joined the army連結並當副詞修飾動詞joined，例如：This is the park where we played baseball last night.
3. 關係代名詞、關係副詞是截然不同，不可混淆。

when/ which兩個字都是關係代名詞。

1. which Joseph shall never forget是一個關係子句，用來形容"the day"這個字。

The day which Joseph shall never forget.翻譯：Joseph絕對不會忘記的一天

2. when he joined the army是一個關係子句，用來形容"the one"這個字。

The one when he joined the army.翻譯：他從軍的那一天。

(因為先前已提過the day, 所以用the one代替)


The day which Joseph shall never forget is the one when he joined the army.翻譯：

Joseph絕對不會忘記他從軍的那一天。


關係代名詞用法參考資料如下，有興趣可以參考一下~~
http://blog.sina.com.tw/missrich/article.php?entryid=581717