Only on tuesdays...
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It sure is
no
No you cannot. You can only catch Dialga in Pokemon diamond and platinum. Although you can only catch palkia in pearl and platinum.
Beat the Pokemon league again and go back to spear pillar and you will see dialga again.
No.
After Palkia you get Dialga
yes
Trade with Pokemon Pearl or Pokemon Platinum
giritina, palkia and dialga
it's impossible, and I have action replay
Spear pillar