Learn Italian: Per tradizione, come si festeggia il matrimonio qui? - By tradition, how does one celebrate a wedding here?

No one in particular! The si here is the impersonal si. It is used to talk about people in general. In English, we usually translate this using 'one', 'people', or 'they' (as in 'how do they celebrate a wedding here').

Why is the verb festeggia in the singular form?

When the impersonal si is followed by a direct object, the verb has to agree with that object. Because il matrimonio (the wedding) is singular, we use the singular verb festeggia. If the object were plural, like i matrimoni (the weddings), the verb would change to the plural form: si festeggiano.

I thought matrimonio meant 'marriage'. Can it also mean 'wedding'?

Yes, it means both! Italian uses il matrimonio to talk about the lifelong relationship (marriage) as well as the event and party itself (the wedding). Because of the verb festeggiare (to celebrate), it is clear that this sentence is asking about the wedding celebration.

Why is the preposition per used in per tradizione?

Per tradizione is a set phrase that means 'by tradition' or 'out of tradition'. Italian prepositions often don't translate word-for-word into English, so it is best to learn this as a single vocabulary chunk. You could also replace the whole phrase with the adverb tradizionalmente (traditionally).