Learn Farsi: در فلان مهمانی، اشراف در عمارت جمع شده بودند. - At a certain party, the nobles had gathered in the mansion.

فلان is used here as a determiner modifying a noun, which is why it comes right before مهمانی (party). If we wanted to refer to a person as a standalone pronoun like 'a certain someone', we would use فلانی instead.

Usually فلان means 'such and such', so why is it translated as 'a certain' party here?

While فلان often means 'such and such' when you forget or just choose not to specify a name, in storytelling contexts like this one it translates more naturally as 'a certain'. It sets the scene by referring to a specific but unnamed event.

I notice the verb جمع شده بودند is plural. Is اشراف (ashrâf) a plural noun?

Yes, اشراف (nobles) is an Arabic broken plural, meaning its internal structure changes instead of taking a regular Persian plural suffix. The singular is شریف (sharif). Since it refers to a group of humans, it correctly takes the plural verb ending in بودند.

What tense is جمع شده بودند and how is it constructed?

This is the past perfect tense, translating to 'had gathered'. It is built using the past participle of the compound verb (جمع شده) followed by the simple past tense of the auxiliary verb بودن (to be), which is conjugated here as بودند to match the plural subject.