go to veilstone city and go to the galatic headquarter all the way north of the city.
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you have to go to lake verity first then go to acuity.
Mars does not appear at lake acuity. If you beat mars at lake verity then go to the upper part of mt coronet and explore until you reach snowpoint city. If you're at lake acuity and beat Jupiter then go to the galactic HQ in veilstone city.
After you go to lake acuity you must go to mt. coronet and catch giratina. After that,the major blackout in sunyshore city finishes off and you go there and battle the gym leader.
go 2 spear pillar
go to snowpoint city ;)