There isn't a date for the Arceus Event in Pokemon HeartGold (unless you mean the event where you get Arceus). When you send over any Arceus into your HeartGold game, the event is activated. You'll have to go to the Ruins of Alph in order to do the event.
An event
Through an event.
Yep
You can't.
there is a Nintendo event maybe at gamestop and go to it with Pokemon heartgold and soulsilver and there you will get celibi
To get a Mew in Pokemon Heartgold, you could do it through a Nintendo Wi-Fi event.
you can get celibi on heartgold only from a wi-fi event
witch event
you get it by an event.
Nintendo event
Get the celebi event.