Question

After you defeat Reshiram the first time what day does it come back?

Answer 1

Just defeat the elite four and he will respawn, but of course you will still have to meet the requirements of him appearing (You need to have a Pokemon at lvl100 on your team while you soar with Latios or Latias).

Source: Knowledge

Comments

All of those requirements are met and I defeated it, but  it takes forever to come back

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Try waiting a day after you have defeated the elite four and if that doesn't work then I'm stumped and I'll hide my answer.

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I went back every day and it still hasn't shown itself except on certain days