There are a lot of events on Pokemon Platinum.
as long as the event is still going on, there is a way. it involves getting the same "event gift" in Pokemon diamond/pearl and trading to Pokemon platinum.
Pokemon event
arceus is an event pokemon.
Event
by event.
Event
there is no ho oh or lugia event in Pokemon platinum
deoxys is an event pokemon. that event passed forever ago
The arceaus event
none
There Will Be A Upcoming Pokemon Event And It Is About Jirachi.
Event