The word "do" is commonly used as a verb, however, it can also be used in an informal way as a noun, to refer to a party or social event.
The office has a Christmas do every year.
Are you going to Anna's leaving do on Friday?
They've got so much money, I'm sure they'll have a big do when they get married.
Have you been to any dos recently?
The sources I looked at said this term is mainly used in the UK, so I'm curious to know if people from other English speaking countries are familiar with it.