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.
Through an event.
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.
Get the celebi event.
you get it by an event.