Learn Swedish: Chefen var på minglet igår. - The boss was at the networking event yesterday.

This follows the "Place before Time" rule. When you have phrases telling you where and when at the end of a Swedish sentence, the place (på minglet) almost always comes before the time (igår).

Could I start the sentence with igår instead?

Yes, you can! But don't forget the V2 rule, which says the verb must be the second piece of information. If you start with the time, the verb var must come right after it, swapping places with the subject: Igår var chefen på minglet.

What gender is the word for "networking event"?

It is an ett-word: ett mingel. Since the sentence is talking about the specific event, we add the definite ending -t to make it minglet. It's a loanword from English, but it acts just like a normal Swedish noun.

Why do we use the preposition på here instead of i?

Swedish uses på for events, gatherings, and activities. Because a networking event is a social gathering rather than a physical container or room you stand inside of, you are på minglet.