I quote only the first two stanzas of the poem as follows.
Ob ein Gott sei? Ob er einst erfülle,
Was die Sehnsucht weinend sich verspricht?
Ob, vor irgendeinem Weltgericht,
Sich dies rätselhafte Sein enthülle?
Hoffen soll der Mensch! Er frage nicht!
1 Die du so gern in heil'gen Nächten feierst
2 Und sanft und weich den Gram verschleierst,
3 Der eine zarte Seele quält,
4 O Hoffnung! Laß, durch dich empor gehoben,
5 Den Dulder ahnen, daß dort oben
6 Ein Engel seine Tränen zählt!
Is Die in the nominative or the accusative case?
Is du a reference to Hoffnung or not?
To set the questions in context, I will describe how I have tried to understand the second stanza.
I assuming that the first three lines form a relative clause that attaches to Hoffnung in line 4 (and that line 3 is a relative clause attaching to Gram in line 2).
When I read only line 1, the syntax seems unproblematic. We have someone, being addressed as du, who celebrates hope on Christmas nights, or grammatically speaking Die is in the accusative case and du in the nominative.
But that hypothesis is made untenable by line 2. If Die is still in the accusative, then verschleierst seems to have two accusative objects, i.e. Die and Gram.
One way to give verschleierst only one accusative object is to put Die in the nominative.
But if so two problems seem to arise in line 1. First, what is du? Is it apposite to Die? Which might give us something like:
O hope! Who, o you, so gladly celebrate on Christmas nights and gently and softly veil the grief, which torments a delicate soul.
Second, what does it mean for hope to celebrate on Christmas nights (or at any other time for that matter)? Does it mean that he who celebrates is in a state of hope, or that it is hope that permits anyone to celebrate? (A rather pessimistic worldview as one might think one celebrates what one has, not what one merely hopes for.)
If, as I have ventured to guess, du is setting the grammatical person of Hoffnung (as second), please comment on how common it is to use a relative pronoun followed by a personal pronoun for this purpose.
For anyone interested, I found the full text of Urania, the work in which the quoted lines occur. See page 15.