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.
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Technically,yes you can get Arceus in Pokemon Heartgold,but only by trading it to your Pokemon Heartgold game from Pokemon diamond,pearl or platinum.
There is currently no Arceus event for Heart Gold, but there was an event for DPP where you could obtain Arceus to level 100 with the moves Spacial Rend, Roar of Time, Shadow Force, and Judgment. Currently that is the only way to legally obtain Arceus.
there is not going to be an event for arceus on Pokemon black sorry to disapoint you.....!
You can only get Arceus by an Event.
i don't think Pokemon will have an arceus event. i think its just a hack Pokemon or a super secret Pokemon