Marcus prorsus obliviscitur quo die comitia habeantur.

Because obliviscitur comes from obliviscor, which is a deponent verb.

Deponent verbs:

• have passive forms
• but active meanings

So obliviscitur is grammatically shaped like a passive verb, but it means he forgets or he is forgetting, not he is forgotten.

Its dictionary form is:

• obliviscor, oblivisci, oblitus sum = forget

Obliviscitur is:

• 3rd person singular
• present indicative
• from obliviscor

So it means:

• he forgets
• or he is forgetting

Since the subject is Marcus, the form must be singular: Marcus ... obliviscitur = Marcus forgets.

Prorsus is an adverb meaning:

• completely
• entirely
• absolutely

So Marcus prorsus obliviscitur ... means that Marcus does not just forget a little — he totally forgets.

It modifies the verb obliviscitur.

Quo die means on what day.

Breaking it down:

• die = ablative singular of dies = day
• quo = ablative singular of the interrogative/relative adjective, agreeing with die

So literally it is:

• on which day
• or in smoother English, on what day

The ablative is used because Latin often expresses time when with the ablative:

• eo die = on that day
• quo die = on what day

Yes. It is an indirect question.

The direct question would be something like:

• Quo die comitia habentur?
• On what day are the elections held?

After a verb like obliviscitur (forgets), Latin often turns the question into an indirect one:

• Marcus obliviscitur quo die comitia habeantur.
• Marcus forgets on what day the elections are held.

So the whole clause quo die comitia habeantur functions as what Marcus has forgotten.

Because Latin normally uses the subjunctive in an indirect question.

So:

• direct question: Quo die comitia habentur?
• indirect question: ... quo die comitia habeantur

Here habeantur is:

• present subjunctive
• 3rd person plural
• passive

The present subjunctive fits well after the present main verb obliviscitur, since the action is viewed as happening at the same time.

Because the subject of that verb is comitia, which is plural.

So Latin uses:

• comitia habeantur = the elections are held

Even though English may sometimes treat something like the election as singular, Latin comitia is a plural noun, so the verb must also be plural.

Comitia is a Roman political term.

It refers to:

• a public assembly
• especially an assembly for voting
• often, in context, elections

It is a neuter plural noun.

So depending on context, comitia habeantur may be translated as:

• the elections are held
• the assembly is held
• voting takes place

In a sentence like this, elections is often the clearest English translation.

Latin commonly uses habere with comitia in the sense to hold elections / to hold an assembly.

So:

• active: aliquis comitia habet = someone holds the elections
• passive: comitia habentur = the elections are held

That is why the passive makes good sense here.

Yes, Latin can use quando for when, but quo die is more specific and a bit more concrete:

• quando = when
• quo die = on what day

So quo die emphasizes the exact day.

The whole clause acts as the content of what Marcus forgets.

In English, we often say:

• He forgets when the elections are held
• He forgets on what day the elections are held

Latin does the same thing by using an indirect question after obliviscitur.

So instead of a simple noun object, the verb has a subordinate clause as its complement.

Not really. Latin word order is flexible, but this sentence is quite natural.

• Marcus = subject first
• prorsus = adverb placed near the main verb
• obliviscitur = main verb
• quo die comitia habeantur = indirect question following it

A very literal order is:

• Marcus completely forgets on what day the elections are held.

Latin often places the subordinate clause after the main verb, just as here.

Because here quo is agreeing with die, not standing by itself.

On its own, quo can sometimes have other meanings depending on context, but here:

• quo die = on which day
• not where

The noun die makes it clear that the question is about time, not place.