When are meet the baby parties? (At what age)

And what do they typically entail?

I was thinking about having a joint meet the baby and birthday party for my toddler since their birthdays will be the same month. Im okay with my sons party being weeks-months past his actual birthday.

I have no idea what they actually are but have seen a lot of people post about meet the baby get togethers so I figured I could get some help.