Cum sol occiderit, convivae in atrio sedebunt et convivium incipient.

Here cum means when. It introduces a temporal clause:

Cum sol occiderit = when the sun has set / when the sun sets.

Latin cum can mean other things in other sentences, such as since, because, or although, but in this sentence it is simply marking time.

The form occiderit can look the same in both tenses, so context is what tells you which one it is.

Here the main verbs are clearly future:

• sedebunt = they will sit
• incipient = they will begin

So occiderit is best understood as future perfect indicative: when the sun has set. The idea is that the sunset will happen first, and then the guests will sit and begin the banquet.

If this were a different kind of cum clause, especially in past narrative, the subjunctive would be more likely.

Latin often uses the future perfect in a subordinate clause when one future action will be completed before another future action happens.

So the sequence is:

1. the sun will have set
2. the guests will sit and begin the banquet

That is why Latin says Cum sol occiderit, ... sedebunt et ... incipient.

English often does something similar in meaning, but not always in form. We usually say When the sun sets, the guests will sit... or When the sun has set, the guests will sit...

Yes, both are future.

• sedebunt = future of sedere
• incipient = future of incipere

They look different because Latin forms the future differently in different conjugations.

• 1st and 2nd conjugation often use -bo, -bis, -bit, -bimus, -bitis, -bunt
• 3rd and 4th conjugation often use -am, -es, -et, -emus, -etis, -ent

Since incipere is a 3rd-io verb, its future plural form is incipient, not incipiunt. So here it definitely means they will begin, not they begin.

Convivae is nominative plural, and it is the subject of both sedebunt and incipient.

So:

• convivae sedebunt = the guests will sit
• convivae convivium incipient = the guests will begin the banquet

A learner may notice that -ae often suggests a feminine first-declension noun. But conviva is a noun of common gender: the form is first-declension, but it can refer to male or female dinner guests.

Because in takes different cases depending on meaning:

• in + ablative = in / on a place, showing location
• in + accusative = into / onto a place, showing motion toward

So:

• in atrio = in the atrium / in the hall
• in atrium would mean into the atrium

Here the guests are sitting in a place, not moving into it, so the ablative atrio is correct.

Convivium is accusative singular. It is the direct object of incipient.

So:

• convivium incipient = they will begin the banquet

A useful pattern to remember is that verbs like incipere often take a direct object:

• librum incipit = he begins the book
• bellum incipiunt = they begin the war
• convivium incipient = they will begin the banquet

They are related in meaning.

• conviva = guest at a banquet, literally someone who lives/dines together with others
• convivium = banquet, feast, dinner party

Both come from the idea of living together / dining together. Latin often has related nouns like this, so the similarity is not accidental.

Latin word order is flexible, so the sentence could in theory be arranged differently. But putting the cum clause first is very natural.

It does two things:

1. it sets the time frame before the main action
2. it gives the sentence a smooth narrative flow: first sunset, then the meal

So the order is not strange at all. It is a very common Latin way to present events.

In this sentence, with sol as the subject, occidere means to go down / to set.

So:

• sol occidit = the sun sets

There is also another very similar verb, often written the same way without macrons, meaning to kill. In dictionaries these are usually distinguished as different verbs. Context tells you which one is meant, and with the sun it is clearly set, not kill.

The most natural English versions are:

• When the sun sets, ...
• When the sun has set, ...

The Latin is literally closer to when the sun has set, because of the future perfect occiderit. But in normal English, when the sun sets is often the most idiomatic way to express the same idea.