You come to PokéMart where already green deliveryman (events) is awaiting you.
You save your game.
You obtain eg. Mew.
Mew you obtained is Adamant nature, however, you want it be special.
Can you load your game and try it as much as you want?
For many event Pokemon, you can reset for their natures. For the shiny Dogs currently given out, they have set natures. Most of the Pokemon sites that carry news of events will note which events have random natures and which are already set.