Seize the Day

You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

Seize the Day can target a creature that's already untapped.

If the target creature becomes an illegal target for Seize the Day, the spell doesn't resolve. There isn't an additional combat phase or main phase.

If you somehow cast Seize the Day during an opponent's main phase, that opponent attacks during the resulting combat phase, since it's still that player's turn. If you somehow cast it not during a main phase at all, all that happens is you untap the target creature.